In my experience I have come across people who presented as "stupid." I have worked with rotating casts of temps at a couple of jobs - the "stupid" people are just as common in management as in the temps.
Usually I think it is because they are not present in their moment/don't care about what is happening, so they aren't really stupid but are just... vacant. Once you get them talking about a project they are actually passionate about, you can see the light of their intellect. Some folks however really do have a hard time understanding the world through logic, even when they are really trying.