There's an ambiguity to the nature of the entire Kirby series, placing each game within "Dream Land" (though this appears to be an invention of NoA's 1992 localisation team, who didn't want to stick with "Pupupu Land" for whatever reason), but it is explained that Earth and Planet Popstar both exist on the same celestial map.
Kirby's Dream Buffet has the best chance of being a dream, as rolling around enormous cakes seems like the kind of thing Kirby would be inclined to dream about, but the game's intro shows this was the result of him picking up a magical fork. Nevertheless, food for thought.

Haven't played it so I may be remembering some other 360-era JRPG, but Eternal Sonata?

yeah eternal sonata takes place within chopin's dream while he's dying of tuberculosis

Dreaming Mary! 0:

Heart of Darkness, Monkey Island 2, Star Ocean 3 I think?

how could you forget Keen Dreams

Morpheus (1998), the worst ending in Silent Hill 1, Silent Hill: Shattered Memories, Sanitarium, Weird Dreams, and Nightmare Ned.

I think Star Ocean 3 is a pretty big stretch.... but if you're going to include it then Danganronpa V3 might also fit

I don't think Dream Buffet belongs on this list

gloirous list brother!

Klonoa 1 and 2

NiGHTS into Dreams...

Drawn to Life: The Next Chapter