Maybe I'm wrong (also not an American, so I haven't been as exposed to Trump as most Americans have) but my impression is that he's not dumb, he plays dumb.
What's more likely? Being lucky enough times to keep his business going, or being veery veery cunningly business-savage deep inside, yet do the stupidest things (start businesses with no expertise, no secret niche, no partner - vodka, flight, university).
He's a salesman, a snake charmer, like Reagan, the guy who keeps on going, persistent, strong, never defeated, because it's not how many times you fall, it's that you get up that many plus one times, and sell, sell, sell, do the talk, push, make deals, don't worry, there's always a next door where you can try a new pitch and dump that vacuum cleaner.
I still can't believe someone like him can be president. If Jeb Bush had won, I would have been a little disappointed, but I would have gone to sleep and not worried that much about it. But he's terrifying. Maybe he'll just be unable to accomplish anything, so he will self-block, doing as much as obama with republican congress. More likely is he won't be in touch and pence can do whatever he wants.