I don't remember people thinking HW Bush was dumb. Or McCain, or Romney, or Ryan, or McConnell, or even someone like Gingrich. Quayle, Palin, W Bush (very incorrectly, dude was wrong and/or lying about a lot of stuff but he wasn't dumb), and Trump, sure.

The thing those people have in common is that they have unorthodox public speaking styles. Especially Trump. It's kind of a pro wrestling adjacent style -- lots of performative bombast, specific tropes referencing cultural touchstones, I'm not trying to change anyone's mind on any substantive issue. I'm just trying to put myself into a particular box in the viewer's mind. It can be effective, but when it's not, it comes off as buffoonish.