Every single tech company unveiling is a guy in a tshirt/sweatshirt and jeans ever since Steve Jobs did it.
You could probably say Zuckerberg was the real pioneer of the look. Recall that Steve Jobs still looked like business casual (albeit with jeans and white sneakers), but it was classy in a way. Straight up t-shirts are a different area.
And frankly, I don't give a damn what anybody wears, even if they are financial investors. Demonstrate to me you know what you are doing, and what you wear has no bearing on me. If you look sloppy, I'll probably be eyeing you suspiciously until you prove I can trust you; if genius level, cool, don't care about your looks. If you are sloppy and a slacker and behave as such, it'll show in more ways than just what you wear.