Flat earth, Q and friends are not part of the endemic beliefs of the general population. These are fringe beliefs. People might have the wrong impression about their spread because people like to write a lot about them. Curiosity makes luminaries waste out time or offer us entertainment. Also attractive to people that aren't too sure about themselves and try look for reassurance while looking at the clearly absurd. "At least those are more stupid than I am", "If those are not afraid of X, why should I be?".
Ukip, Brexit, AfD, Trump, FN are different phenomena in my opinion.
Some of Brexits arguments were objectively correct and there are no certainly correct or wrong opinions here. Especially on hot button terms like sovereignty and self-determination. It depends on what level you look. Self-determination against the EU or Russia/China/US? There are different answers here and none of them is entirely true. But the opposition just shut them down, so people responded in kind. That it was basically 50:50 is surely accidental, but I like to see it as a hint.
The AfD is radicalized now, but it was firstly headed by someone that has been vindicated by projections other economists failed at. He isn't in the party anymore, but what do you think happened with those that shared his beliefs? Those were correct too, what do you think they believe about their opposition today?
These beliefs aren't mine, but I understand their distrust. In many cases it is distrust that was earned, not by their failure to form an opinion, quite the contrary their opposition was in many cases completely incapable to understand the nature of their opposition, which paints a shadow on their own judgements.
The indignation against people not being able to think is misplaced in many cases, that is also true for the other way around of course.