It might be cute for a character, and I may well use it in a story some day, but I don't think I'd ever call my kid Pythagoras - he'd be in for some serious teasing when his class started learning the Pythagorean Theorem, or learning about Pythagoras himself. Plus it's quite hard to get good nicknames from, is hard to spell and pronounce and sounds (if I may say so) a little bold and presumptuous for me.