tl;dr Overproduced, misguided, focused on difficulty over the value in how the original delivers its message in both gameplay and writing, bad writing, fanservicey

To sum the experience up, Undertale Yellow feels like Undertale but without the soul that made the game stand out and become one of the best games of all time, and instead trying to fill that in with just overproduction.

Did you like Undertale for the characters being fleshed out and given just the right amount of time to actually add emotion to the moral choices of killing and sparing or the world feeling full enough that going through pacifist felt rewarding? Tough luck, the characters barely get explored, and when they do, it's either an archetype that was similarly done in Undertale and then beaten over your head anyways, or sprinkled into stuff that you won't encounter until you really can't be bothered to care anymore.

Did you like Undertale for the decent RPG gameplay balanced with a moral mechanic? Well, too bad! because now it's been "improved" by upping the difficulty and complexity of even the basic enemy encounters, defeating the whole point of the very carefully balanced system that the original game had, where a neutral route has a pretty average experience, pacifist comes with more character and joy and generally happier endings, but comes at the difficulty, because making the morally right choice is sometimes HARD, and genocide being the note on just grinding mechanics in RPGs in general, sprinkled in with some superbosses, You know, something nuanced, creative, deep.

Instead of that you're getting the rose tinted fan view of that idea, where the game is a means to an end for an overproduced superboss no matter which route you take, and worldbuilding and characters being more of an afterthought, I always thought it was a really cool part of the story being that you weren't seeing merely a slice of the Underground, you were seeing a decent chunk of it, They're oppressed, cramped, scared, trying to just live anyways, under the hopes that Asgore will free them with the seventh human soul, but no, Instead you get multiple new areas bursting with new characters, and even near the end implies that there's EVEN MORE to the underground you haven't seen, it's totally pointless.

Major characters are a massive blunder, you have Martlet which is just an okay crack at the type of personality Papyrus does in Snowdin, but it lasts like no time at all before you're split off and yet the game still expects you to care about them, there was no scene with setting up the puzzles and having amusing gags, there was just flavor text saying "The puzzles are a bit fucked up LMAO" Nothing really changes with her, but there'll be another note in the genocide ending section.
Starlo is just... another type of Papyrus character who's a bit over the top but intended to be endearingly so, it's the other half of his character, Not really much of note, it's similarly endearing and the scenes with coming to terms with his emotions are nice, even if they are predictable.
Ceroba, I thought she was just a neat character to have as an addition to Starlo, but then after you leave the dunes she's... following you... This can't end well, She ends up just being a bit of dialogue to constantly hammer into you "MY HUSBAND WORKED IN THIS LAB, THE LAB IS SHUT DOWN NOW" or "I AM UPSET" Not a fan of her design, and it only feels like a means to an end for her role overall in the Pacifist route, being an overdesigned superboss (surprise! her character was that she has a DEEP DARK SECRET!)

Neutral, you go through and suddenly Flowey says "WAIT YOU'RE GOING THE WRONG WAY, SUPERBOSS AND THEN ENDING NOW" and then finishing it off with "Okay now play the pacifist route"
Mediocre ending, Boss fight was really cool though excluding the annoying creepypasta level stuff and the glitch filters.
The heart of Undertale's neutral ending rests in the fact that it is still a full on ending, life goes on, it's a proper ending with a proper epilogue, though being fully text-based is supposed to make it feel a bit, bittersweet, to make you think "Could I have done better?" And then Flowey is supposed to be the voice to the player telling them yes, let's see if we can do better.

Pacifist, It's pretty much the same deal as neutral but with more Ceroba, because they wouldn't want their star of the show to have a dull fucked up backstory.
Long story short she's an idiotic hypocrite but since she was almost surely created just for this moment, she has to overpower all your friends and want to KILL YOU!!!
And then begin overproduced superboss fight.
Afterwards you get a weird mildly satisfying ending, but it struggles hard because they REFUSE to interact with anything seen in the base game, so it ends off feeling really shallow and more of a pointless tug on the heartstrings.
"u sacrificed urself... because then the monstres can live..." Why not have the neutral route be the canon ending to Undertale? Have the previous 6 humans be a little more trigger happy, to make it as reasonable that the monsters are afraid of the humans, why people like Undyne are around, why the Royal Guard exists, etc. and then let the Pacifist ending in Yellow be a "what if" where Clover just lives on, doing their best to help the monsters, and when the day comes that the Eigth human falls down, they can help them and potentially unite the underground, or find a way to pass through the barrier and destroy the divide forever, send home that Toriel was right, they could have found a way if they strived for peace instead of just trying to protect their own kind in fear.
EDIT: After a bit of extra research, it seems like this is just here because the other outcome to this route is killing Ceroba and then fighting Asgore, same end result but fits in with Undertale a lot better, no idea why the fully pacifist route doesn't end similarly, the reflection into self-sacrifice part is just a bit corny, I assume it's trying to convey the whole "Justice" thing but it isn't doing a great job at it.

Genocide, bad, major identity crisis too.
It can't decide if it's supposed to mimic the "oooh creepy Chara" stuff or the cold fist of justice, so it just does both whenever it feels like, but by trying fails at both.
If you're going for the Chara thing, skip the puzzles, the whole point is supposed to lampshade the culture of optimizing the fun and character out of a game, a mindless killer trying to be as powerful as possible, you know, the player CHARActer, but instead you have Clover, the angsty evil child.

Could have something going with the whole justice thing, Clover is looking for the 5 missing humans, and it could be treated as trying to lampshade the idea of "self defense" where if Toriel had another minute she would have told you, if a monster initiates battle "Try starting a conversation with them" and instead running the route with the idea of "Okay, most people going through this know what Undertale is about. So let's dial back the you're a monster part and leave the morals less written on the sleeve, less supernatural, more "who's really in the wrong here" and that could have been done by removing the genocide route, being totally out of character for Clover, and instead being more of a "Self-defense" route, where you're killing anything you come across, but not grinding them.

But nope, Clover the angsty evil child (with a gun) proceeds to go around murdering everyone they can find, and the gameplay isn't much better to make up for this, you g et to pick from 1 of 3 things, Hard mode fights, Pitiful bosses that can't do anything to you, and, you guessed it, MORE OVERPRODUCED SUPERBOSSES!

What's that? You enjoyed Sans and Undyne the Undying because it's a sad turn for the two protector characters of the game, trying to be major roadblocks, and Sans revealing that he knows what's going on behind the scenes? You thought those were cool moments that expanded on the characters?
Great! Here's a recycled fight from pacifist, now easier, who is meaner to you and that's it, there's your Undyne!
Aaaand here's some random shmuck who didn't get enough development in any route, now she's UNDYNE THE UNDYING's design and determination, mixed with Sans power level, Nothing interesting happens character wise, and then you just fucking nuke Asgore for no conceivable reason other than "U R SUPWER POWERED NOW CUZ U KILLED A REALLY STRONG PERSON" and then they're like "Oh shit wait a minute, this isn't a Chara thing" and rush to say "UHH CLOVER WAS DOING IT BECAUSE SHE WAS AVENGING THE FIVE HUMANS AND THEN SHE LEFT THE BARRIER NOW WITH THE HUMAN SOULS FREE MONSTERS DEFEATED FOREVER" which could have been an interesting route to take, if there was ANY reference to this at any other point in the game other than one of your starter items being the missing poster.
And to top it all off, they can't even make the character deaths sad, they're just, gross. There's an attempt to make you feel bad by having them suffer when they die and melt more gruesomely than Undyne did, but it just feels like a shitty creepypasta story, I don't feel bad when Super-Martlet melting disgustingly says "HELP ME" I felt bad when in her last moments, Undyne was still smiling confidently, her character still showed, it's the same character that you fall in love with in the other routes, but pushed to their limits.
Sure I feel a bit bad when the dance guy just gets ignored then murdered, but he's of no impact to the rest of the story.

Onto the nitpicks, Spritework is inconsistent as all hell, They try to stick with the undertale style, basic sprites, detailed but black and white battle sprites, which is good! but then as you go on, sprite complexities in both modes increases, leaving the older areas looking bad and outdated, they branch away farther from the zones of Undertale, and even make a massive sin of animating the sprites heavily, a part of the humor was with how basic the sprites were, seeing some movements were just funny, being lifted by a bird and you're just in the same pose, and it added a lot of impact to when a character was actually animated, like Asgore taking out his weapon from under his cloak, it's not something you see all the time, but instead basically any advanced movement is animated, RUNNING is animated, it's jarring going from a NES spritework walk to a "So retro!" Indie game run animation, It's something I feel Toby probably learned from Andrew Hussie, where in Homestuck a lot of quick little visual gags were something like a basic still sprite for a character wobbling around and falling over, or bouncing around all over the place, etc.

Clover interacts too much, The whole point of the silent protagonists is that they don't talk, outside of the basic decisions obviously, but they're supposed to be so basic you might as well not be talking, it's a game about tackling RPG tropes, you are a vessel for the player, and having a lot of moments where Clover goes in for a hug, or waves back at someone or something like that, it does both a disservice to the humor of the game like mentioned above, and just feels disconnected from the style of Undertale.

And another point on the writing, it just fails to be funny in general, it lacks that writing style, that self-referential humor seen in Undertale and Homestuck, and there's not even any hard hitting lines, nothing like the scarier lines from Genocide in Undertale like in New Home, or the famous "Despite everything, it's still you." It just feels like fanfiction made by people who loved the characters and gameplay of Undertale, but not people who loved the whole package, or even understood it.

I was just not fond of the soundtrack, it was weak minus a few parts and I feel this comes heavily from being too ambitious with the soundtrack, everything, even the stuff that heavily carries motifs of Undertale tracks has a lot more going on, more advanced soundfonts, it needed to be dailed back.

Lastly, balance was just fucked, throughout the middle of the game it was all over the place, then in all but Genocide (because it's always fucked up difficulty wise and that's the point) you're smacked with a sudden superboss that just grinds the game to a halt, and doesn't play fair like Omega Flowey or Asriel.
Sometimes bosses would just have suddenly bad difficulty curves in their own fights, having like 2 moderately difficult attacks, and 1 that can easily just be AFK dodged, and then putting you in one where you're likely to be hit 3-5 times in a row only 30% into the fight, forcing you to restart and make sure you're max health so you can understand what the hell is going on for the next attempt, in general it felt like a lot of the fights just wanted you to be juggling health items.

But then, in the final boss in the steamworks areas, you get NO VENDORS, You are told to get fucked if you wasted your healing items beforehand, because you can't backtrack, It was a huge pain in the ass and I have no idea that oversight didn't get noticed.

Regardless, after all of my complaints for the game, I think for any Undertale fan, you should at least play it, it's competently made enough that you'll probably at least come out thinking something, which is far better than I can say for the mods that are literally just boss fights, or just not having anything like Undertale instead.