I'm not so bothered by the "cutesy, kiddy" thing than I am of the idea of playing a Mario game in first person. It's almost like taking Mario out of the game with no immediate benefit or reason. A game like Metroid Prime made sense in first person because Samus already uses a gun/arm-cannon. Even though Mario has been in so many spin-offs, a paintball game just seems forced to me. Not saying it can't work, I just don't understand the appeal.
If Nintendo insists on releasing a non-violent FPS, they can certainly do so without using Mario (e.g. Miis) and especially without having Retro Studios, of all teams, developing it. If Retro Studios were to develop a Mario game, I'd much rather it be a 2D side scroller considering how amazing Donkey Kong Country Returns was. 2D Mario could certainly use a heavy dose of imagination not really seen in the New series.
My hope is that Retro Studios uses its immense talent on something that reaches a lot of people and reminds us all how good video games can be, something that isn't just paint-by-numbers or following trends. As I've said before, Metroid Prime was one of those games that changed the way I look at games; it set a new bar. I think Retro Studios still has it in them to make a game with the same impact. It's good for Wii U and it's good for the industry.