I don't think Mario games are kiddy at all. They are well crafted games and can be downright devilish in difficulty for more experienced gamers.
I wonder what those who say it's too childish want. I'm curious. Is it blood, gore, and M-rating, for Mario? How many less copies would Nintendo sell of an M-rated Mario! A story in Mario is inconsquential, it's all about the gameplay. Not deep narratives. Perhaps folks want M-rated Mario with codec-sequences between warp pipes and long CG cutscenes, I don't know.
Mario is for EVERYONE.