I think Joey was told in the early years to really use this 'stupidity' as an identity almost. He's not as stupid as some of the things he comes out with; he plays on it massively. But I do think now he's a little older perhaps not so much...
But he definitely had his own mental health issues, he's got dyslexia and he also just isn't a grade A kinda student!
His whole team have pushed this stupid persona for him over the years for publicity. Wild really.