I think a lot of people said "he isn't very smart" when what they meant was "he isn't very stately". And his public persona wasn't.
His verbal gaffes are well known, even if they are overblown by the press. He sometimes spoke with a drawl. He was prone to making facial expressions that looked like confusion, even when that probably wasn't the cause. He embraced the whole Texas cattle rancher/farmer image with the cowboy hats. I think his sense of humor seemed a little pedestrian to some.
None of things really have much to do with intelligence other than it's not typically how we envision intelligent people.
His verbal gaffes are well known, even if they are overblown by the press. He sometimes spoke with a drawl. He was prone to making facial expressions that looked like confusion, even when that probably wasn't the cause. He embraced the whole Texas cattle rancher/farmer image with the cowboy hats. I think his sense of humor seemed a little pedestrian to some.
None of things really have much to do with intelligence other than it's not typically how we envision intelligent people.