Is he mentally handicapped? Like, if Bubby were to receive a proper education instead of being trapped with only his mother, will he be able function like a normal person? Or will he still be talking and behaving like he did in the movie?
...a little fantastic, and fleeting, and out of reach.
If he had a "normal" life, he may not still be the sharpest tool in the shed, but I do think his mental deficiency was due to lack of proper socialization.
There was not even a TV or radio in the room he was raised in for him to learn from. And I don't think mommy dearest cared to spend time teaching him anything except to comply w/her abuse.