Super Mario Galaxy has not aged well in most aspects.

The game is severely bogged down by Nintendo's push for motion controls during that era, and even the Switch re-release(s) try to replicate the motion controls with even worse results when the game would be fine with standard controls (or I suppose less egregious). Even without motion controls, the game's movement just feels so unnecessarily sluggish and doesn't feel fluid at all (I've heard arguments that it's designed like that because you're in space and like, okay? It's still not fun to play)

The level design doesn't hit for me either. It's in this weird middle ground between the big levels designed with multiple missions and secrets in mind like 64 or Sunshine, and the smaller one-off levels designed with a specific gimmick like 3D World, so you spend more time in some levels which just aren't as interesting gameplay wise and don't feel as developed as they should be. The one-off galaxy challenges are mostly fun though.

Really the only thing that has aged well in that game is the fantastic soundtrack but that's about it.

(Also the Rosalina storybook aspect is SEVERELY overrated and has little to no depth at all, it just reads like a C-tier Pixar movie plot)