Based on the fact that despite not being a natural talented rider as Stoner, Marquez or even Martin, not being a super fast rider, still he was able to win a lot of races, recover from very difficult moments and won 2 championships, almost 3 if not by silly mistakes, but won 13 races against Martin 6 in last year on the same machinery. That's the ....... double of the world champion, a really fast guy. You can't do that sort of thing without a high inteligent quotient. You just cannot. He use his adaptative inteligence to compensate his lack of talent and natural skills. I'm not saying he's the smartest ever, no, but among current line up, yes.
Before Bagnaia, we've seen the same with Lorenzo. I'm sure Lorenzo has even more IQ than Bagnaia. Jorge wasn't the kind of guy who had constantly been on a MX track developing riding skills. He didn't do that. Despite that he managed to dominate so much races by a big margin.
You feel when a guy has IQ just listening to him on some ocasions. Take some time to watch interviews and lifestyle videos of Pecco and you'll have the same feeling: hey, this guy really has some brain power.