The game is a story narrated by Rosalina to her Luma children.
Would certainly explain the cutesy storybook format of the whole game. Besides, we've already seen her narrating one of the games to them in Super Mario Galaxy 2's 120-star ending. The game is a metaphor for taking drugs (pot in particular).
C'mon. A "happy tree"? All of those bright colors? Mechanics that involve, basically, the munchies?
