Yes he did. However, if underestimating him helped him win, then that is in the past and we can't fix it now by pretending he's smart.
As you point out, he is not without any skill at all. He understood what it took to win an election better than the rest of us (although he also got lucky that...
I only saw this yesterday (missed it when it was news):
https://mashable.com/2017/10/26/donald-trump-carolin-gallego-new-york-magazine-letter/?europe=true
“Based on the fact that I work for Donald Trump as his secretary—and therefore know him well—I think he treats women with great respect...
I basically assumed that a few years ago. More recently I became less sure and now I'm coming to the conclusion that somehow, Donald is actually less intelligent than George.