Interestingly, I think the fact that he's quite stupid may be how he succeeded at the presidential election. An intelligent, principled person would have a vision and philosophy that guides their actions. When questioned on details or specifics they would use their philosophy to arrive at a reasoned answer.
Trump has essentially no record of this. He has no governing philosophy or sophisticated understanding of how the world works. He seems to do nothing but say what he feels in that moment -- even if how he feels isn't backed up by any reality.
However, in certain election environments that can also be a huge benefit. If the electorate is looking for a foil, someone to reflect their feelings, and the electorate itself isn't acting on calculated reasoning, the dunce with an acute sense of emotional environment is the perfect avatar.
In a sense, the reasoned candidate is at an inherent disadvantage since they will attempt to articulate a particular philosophy. If the electorate is mostly a coalition based on shared emotional reaction, stating a clear philosophy can do nothing but weaken support among your own constituency. In essence, being specific allows people to disagree with you. If you hand wave, they just imagine you think like they do, since they already think you feel like they do.
I can think of another group of people who a similar skillset: confidence artists. They're often not very bright or very skilled, but they are good at reading a room, a person, and an emotion. Interestingly, I don't believe Trump believes himself to be a con-artist; he's just a natural.