There's a part of me that wonders if it's a put-on. His base didn't want an elitist who knows things and tells the truth. They wanted someone who would tell them the lies they want to hear. It could well be tactical ignorance, as much a part of his messaging strategy as hating on Mexicans.
Or maybe he's just really that dumb.
If it does not have anything to do with the Trump Brand---he doesn't care and doesn't know. He is about the worst of us, fortunately subject to Constitutional checks and balances.
He is a moron. Too stupid to be really evil ala Putin.