The thing is that actors aren't actually "dumb". If they were truly dumb, then we couldn't hold them accountable for the things they say. But they're not dumb, they are just ignorant, due in large part to the fact that they work in an industry that:
1) ...is very insular and isolated from mainstream society;
2) ...focuses on "emotional truths" via images and rhetoric to the exclusion of cold hard facts, making them far too easily swayed by images and rhetoric;
3) ...is so lucrative for the few that make it to that level that images and rhetoric easily make them feel guilty for their achievements, and they will latch onto anything that gives the appearance of compassion or altruism;
4) ...is so un-regulated compared with any other industry you can name that retribution against anyone the Hollywood powers that be disagree with (blacklisting) can be done openly and with impunity.
The average actor is probably average to slightly above average intelligence. Their problem is not low IQ but ignorance, isolation, over-reliance on emotions, and a near cult-like loyalty to the hyper-secular ideology of Hollywoodland. In other words, Stockholming on a massive scale...
Dumb and ignorant are bedfellows. I recall hanging out with theatre majors in college (ok, the girls were cute, so dumb didn't matter so much). I was always stunned by their total lack of awareness of the world around them. I was sitting with three theatre majors and it was about 3 months after Nixon had resigned. I made some comment about President Ford and one of them asked me what he was president of and I said the United States. They all looked at me with baffled expressions and one then asked me what happened to President Nixon. None of them knew he had resigned. I SWEAR this is a true story. My own view is that many actors tend to feel guilty for making so much money doing something that takes very little effort. They try to appear more important and relevant than they are so they try to act intelligent by making statements on political issues. They ape each other and what you get is the ignorant leading the ignorant. I recall once when I was a photographer, watching Jane Fonda and a couple of other actresses testify on farm issues before a Senate committee. All except Fonda had been in Farm movies. One Republican Senator asked Fonda what made her an expert on farming and with a straight face she said her Uncle owned a farm. There were titters all around and even the other two actresses were embarrassed because, after all, having been in movies about farming they were the true experts while Fonda was a pretender. Such is the mentality of the acting profession.