I think that fundamentally, there are a lot of gamers that take issue when a game doesn't have a big story with interesting characters.
To quote GameTrailers.
"Super Mario Galaxy 2 is one of those games made for the game player, and not necessarily the game fan. If you feel like you need a deep, involving story with gobs of cut scenes to stay engaged, then steer clear. But if you play video games for what make them different from other entertainment mediums, then put aside a few days on your calendar and get ready for a top shelf gaming experience. There's more creativity in some single stages in this game, than entire other games."
Point is, some people like their games to just be games. Stuff like classic Megaman, Mario, Pac-Man, Tetris, etc, are pure gaming experiences. These days, a lot of people like their games to be gobbled down with other stuff too. Thats probably why some people take issue with it. There's nothing wrong with it of course, but it seems the industry is full of those "cinimatic games only" types.
Also, I think Mario is a great character. At least in his own way. Mario is an average guy (Miyamoto said ugly, but I'd say average), who saves the day. I think that's a great character. No, it's not complex or anything, but Mario is just an everyday guy who does great things.
He's the fireman who gets the cat out of the tree for children. He's the guy that helps an old lady get her groceries to her car. He's the good guy that all of us want to be. That's all he needs to be, no more no less.