I didn't mind the 120 star set-up in Galaxy because the way the levels were done, it was alot more like the 2d Mario game progression than you'd think, especially compared to Mario 64 and Sunshine.
In the latter 2 titles, you'd be thrown into this huge open world and given your goal to achieve, but you could always end up completing a different goal entirely, then have to come back and be thrown into this huge world again to venture off and figure out what you're supposed to do. This was fun the first time around in Mario 64 cuz it was new and exciting, but by the middle of Sunshine it did get a little tedious..
Then Galaxy comes in and you think "Oh great, another 120 stars to collect" but the majority of the stars you had to collect, albeit set in the same galaxy, felt more unique to acquire because they made the experience a little more linear (in a good way). Instead of feeling like you were back in the same ol' Cookie Galaxy (or whatever they were called) and wandering around seeing the same old things, the game made only certain areas/paths accessible and thus created a more traditional 2d mario feel in a 3d mario environment. The stars were simply the end goal in this sense, but instead of going through all of Wold 1 Level 1-8, you gain access to World 2 and its "levels" after only a few stars, which allows you to eventually unlock all the Worlds/Galaxies and experience the majority of the game without having to get every last star.
This is why Mario Galaxy is damn near perfect, and the game closest to reaching SMB3 level of greatness.