no, that's Mario 3DS where the post-game content is worthwhile.
the green stars are simply hidden things to collect throughout old levels. finding and grabbing them is easy and only slightly more challenging than the main game, in that it's still not challenging. It's a lot of "dive off this edge to hit the star, or fall and die" stuff. you do them all to unlock the last challenge stage, which is pretty difficult.
Both galaxy 2 and Mario 3DS have a single challenging level hidden behind all these requirements. Both levels are bittersweet because it shows how the Mario games could be challenging, but it's always only one level.