I think it's the complete opposite - - - Super Mario Galaxy 2 is pretty much the one of the only games that would make no difference whether it was on a more powerful console - It's visuals are absolutely gorgeous just the way they are
There are games like SMG2 that look fabulous on Wii, Donkey Kong Country Returns comes to mind, Kirby's Epic Yarn too - - - - I really think games like these look great the way they are
I do think, however, that most other games would look better on more powerful systems. I think something like Zelda would really benefit from a more powerful console, so would Epic Mickey, The Last Story, Monster Hunter...etc
So I guess my answer is no, SMG2 would not be better on X360