I doubt that our collective intelligence has gone down, because that's kind of an innate thing. But there has definitely been an increase in gullibility, and gullibility is correlated to low intelligence. Some smart people have managed to marshal a critical mass of the gullible, and they now reside safely in a bubble of the misinformation that makes them feel comfortable. Everybody likes to be part of a group, and they will gladly sacrifice some independence to remain part of that group. In the case of Republican voters, that means accepting whatever reasoning the group has accepted as correct, even if it's plainly incorrect.
Biden was a disaster! Was he, though? By most metrics, his administration was very successful. By the end of his term, our economy was "the envy of the world," according to the Economist. Unemployment was low. Inflation was never the huge problem that it was made out to be (not that inflation can be controlled by the president), and we were coming out of the COVID era pretty well, especially compared to the rest of the world.
A mere 100 days later, and the economy is in real trouble because of the U.S.-vs-the-World trade war trump started. Many economists are predicting a recession, a huge (negative) turnaround from "the envy of the world." These are things we can measure objectively, but we all know that those fact-filled arguments fall on deaf ears.
Now throw in an entertaining, charismatic figure like trump, and the gullible masses have found their leader. He galvanizes the gullible base by demonizing minorities and other vulnerable groups (which always works), going so far as to openly welcome white supremacists into the fold (a little trumpian wrinkle on that formula). A little bigotry isn't enough to make the gullible masses leave the safety of their group. Nor are a slew of indictments and 34 felony convictions enough.
Given all of this, I think it's perfectly fair to say that trump voters as a group are of low intelligence.
A bit of anecdotal evidence... I have regular political debates (arguments, really) with some of my old high school classmates. To a man, the trumpies are from the back of the class, the dummies who never answered questions correctly, the ones who struggled mightily just to pass on to the next grade. I think it has been too long since they got a regular, objective assessment of their intelligence. 40 years on, they have learned nothing new, and show no interest in learning anything now, despite wanting to debate. They certainly don't remember learning about the Smoot-Hawley Tariff disaster, nor does trump.