A) Game Overs have nothing to do with difficulty in the 3D Marios.
B) Sheesh, I like hard games but how hard do you want Galaxy to be? I die way more on it than SMB1, 2, 3, World. It's got easily the hardest star in Mario history and stuff like 1-health boss challenges.
It bugs me a little when people call out the Wii entries in the series as being easy, because one of things I admire about Nintendo right now is how even with all the grannies they're luring to Wii, the main franchise games have not gotten easy-fied. Some, like Punch-Out!!, will even rape you.