To be fair people look for mental traits, but nobody can reliably predict if a college kid has them at the level needed to succeed as a pro. Also, if you make a mistake, it's easier to rationalize it if you made it based on measurable physical things instead of psychological factors.
RG3 was supposed to be very smart. By normal standards he probably IS smart. But he never learned a pro offense. Hell, he never learned to SLIDE, so he took linebacker killshots for no reason every game. He had injury problems, but a lot of them went back to his apparently having no football IQ at all - he couldn't make a read and get through his progressions before the defense could hit him, he couldn't make the snap decision of when to step out of bounds or slide to avoid needless hits, and so on.
Conversely, on paper Mariota is kind of the same player, except he's vastly smarter in football terms - but everyone seemed to think he was dumber at the time of the draft. He went from pure college gimmick player to effective pro largely running a proper NFL offense pretty fast. Winson was supposed to be mentally ahead of him because of his pro-style experience, but he seemingly can't stop throwing to opposing DBs at a much higher rate than the "dumb college system player" Mariota.
Heck, the entire narrative around Tebow was about his positive mental traits, everyone was so sure that his A+ work ethic would overcome any problems he had coming out of college, but he actually developed his game not one iota from his time on the Gators. His brief run of success on the Broncos was literally a case of "Give up and run the Gator playbook until the league figures it out." Nobody is going to say he had a bad attitude, but he never learned a damn thing at the NFL level.
Tebow was of course the comparison people made to Dak, and it wasn't meant as a compliment. But mentally speaking, Dak had transcended anything Tebow ever did to improve his NFL game before the preseason was even over. That's a difference so stark that it's kind of stunning that people were equating these two human beings.