You don’t have to be smart to be a conman. And being not the brightest bulb in any room, might actually help Trump doing what he does best (conning people): ethical or non gullible people are very quickly put off by him, leaving him surrounded by unethical people who have no qualms enabling him (as long as they believe that they stand to gain from it) + gullible people that can be milked for all their worth. This is why it was such a smart move for Trump to leave the democratic party and become a republican: the republican party loves gullible voters + unethical fellow travellers. Basically the same reasoning as to why spam phishing mail is so obvious usually: scammers don’t want to waste their time on non gullible people.