I hypothesise a mechanism in the brain that works as a black-box evaluator of idea quality - you feed it ideas and it just gives you an oracular answer of how good the idea is. c.f. the widely-experienced phenomenon where you wake up in the middle of the night with a brilliant idea, write it down and then in the morning realise it's rubbish. This idea evaluator going awry might lead to many an accepted crackpot offer.
Your hypothesis seems to be not the idea evaluator going awry, but the ability to judge its outputs going awry.
This matches up nicely with the process-1/process-2 set of theories, where IQ is just a sort of 'algorithmic' or simulation thinking which is either invoked or not invoked based on one's rational or reflective tendencies. Stanovich extends this to include 'mindware', cached bits of logical or statistical reasoning which the IQ can apply to problems.
The problem with that, and why I didn't mention it, is that we currently have few good ways to measure rational/reflective tendencies, so we would have a hard time finding the possible inverse correlation.
Even i...
Freeman Dyson writes in the New York Review of Books about people who took up the crackpot offer. Not just complete cranks, but eminent scientists such as Eddington who got into crankery in their later years.
New thing I learnt: Dyson was not only a good friend of Immanuel Velikovsky, but considers him a greatly underappreciated poet.