It's hard to say how much of what Diaz says is just trash talk or trying to portray a certain image... But it's clear that at least some of what he says is honest. As uneducated as he obviously is, I respect that he's willing to praise and criticize GSP. The best friends are the ones who are willing to both praise and criticize us, because if someone only gives you praise, or only gives you criticism, then in either case, they're probably not being honest and probably not going to be helpful. Maybe not everything Diaz says is honest, but a lot of it clearly is, and that's what makes him interesting, and what gives me some reason to respect him beyond the simple fact that he's a very good mma fighter.
It seems like Diaz was probably right when he said that he wasn't as well prepared as he should have been, however part of the reason Diaz looked so flat is presumably because George is just a better fighter and specializes in ruining his opponents rhythm.