To quote the great Leon Joworski (who was smart), I would rather have a man of practical insight than the smartest man (in his day it would only be a man). That said, people who are Presidents or leading contenders for President are relatively smart. Really smart Nixon was a jerk (I once lived next to him growing-up and I can say from personal experience he was a jerk), but he was a smart jerk. There are different types of smart, and I guess here people are looking a book smart primarily. For example, Carter graduated near the top of his college class whole playing sprint football, was technically a nuclear engineer while being one of the founder's of the Navy nuclear submarine program, and has won a Noble Prize. Most people would say this in indicative of someone really smart, but most historians would argue he did not have that successful Presidency. Like him or not, Reagan had very little formal education compared to recent Presidents, but got a lot of his agenda through.
Calling Biden or Obama dumb is just dumb. You don't have the success these guys have had being dumb. Both had to overcome significant hurdles to succeed (Biden had a horrible stutter, Obama had to overcome racial sterotypes)