The question is quite tricky to answer. Of course there are quite smart JW or other sorts of religious persons with an IQ above the average. They often have very complex reasons for believing or (acting as believers).
There are a numerous investigations made by sociologist and other researchers, on the subject, many of these essays can be questioned. But in Scandinavia a large group of conscientious objectors, (mostly men), had to make a IQ test, before released from military service.
They scored 5 to 10 points lower than a control group.
Richard Dawkins claims in his well known book, “The illusion of God”, that almost 98% of the nobel prize winners were atheists.
In the congs. I have been, as an elder, sometimes I have found that a large numbers of those I was ordered to take care of in the theocratic school, had great difficulties in just reading. They could not understand what they red.
Of course dumb people are easier to control, from the society’s point of view. They are just told what they should believe, and has no capacities to check out the actual truth in the premises.
So Yes, I think there are more stupid people amongst JW,s , but this is of course just a personal observation, not a scientific essay.
Bugbear