You don't have to be smart to be elected. You have to be a good liar. And it's really easy to be a good liar when you have gotten so deep into bullshitting that you believe your own lies.

Also, being useful to the right people helps. Because they will dump their own money and time into bolstering your campaign.

Based on what is he smart? Every objective datapoint says he's an idiot that fails upwards.

The only thing he was successful in, was literally exaggerating and overselling his capabilities ( eg. He directed the apprentice, it's fake)