The story is that, apparently, he was a pretty good student (AP classes, decent grades), and started majoring in engineering on a scholarship. However, he dropped out partway through because his vlogging was turning out way more money. It seems like most of his behavior is part of his marketing (maximize views), but he claims that his multiple brain injuries while playing football in high school contribute to issues with impulse control.
Here's a video where he explains his process in making the documentary:
Even if he isn't a genius or anything, I think it's pretty clear that he's doesn't have an IQ in the mentally retarded range. I mean, look, this isn't to brag, it's just to make a point, but I'm pretty high up there and it can be difficult to communicate with people whom I know have an average intelligence. (speak slowly, not super coherent in their points, etc.) Pretty sure I'd know if he were actually an idiot rather than, y'know, a guy of typical intelligence trying (and succeeding) at getting viewers.
Now, the fact that America's population loves seeing stupidness on screen to the point that acting dumb has become a marketing tactic... that's a different story.