Nobody's crying foul because of their being some silliness and "slapstick" (I don't think that means what you think it means if you're using it to describe Snow) in a 3D game. They're crying foul because Snow was a complete idiot and was anything but fun.

He was just one of the many characters in that game that never reacts believably to anything that happens. And it's not even just him being in denial, he is literally so stupid that he doesn't realize the trout that's happening to him should bother him a bit. He wasn't a fun loving character keeping a sense of humour in the face of adversity. He was an idiot goof off that didn't seem to notice anything bad was even happening.

Just add him to the pile of half assed writing in that game. The whole thing was a rush job and it shows.

Nobody expects an FF to be 100% serious and nobody wants an FF to be 100% serious. That's not even remotely the problem with the game. If anything, FFXIII spends entirely too much time taking itself seriously which makes characters like Snow and Vanille stand out even more as being so completely out of place because they never take any of it seriously, even when they probably should.