It’s a common mis-conception that pilots MUST be “clever” in the traditional sense.

Having sat the Airline Transport Pilot License (ATPL) exams I would say it’s more quantity than quality. Yes there are a few complex theories to get your head around but it’s mainly a memory test (each subject comes with a hefty textbook and there are 14 subjects to pass).

Flying training itself again has a few complex theories but nothing more than a bit of GCSE Maths - trigonometry is about as complicated as it gets (used for cross-wind and drift calculations).

As someone further up has alluded to, there is much more emphasis these days on human factors ie your personality and how you deal with time pressure, assertiveness, situational awareness, social interaction and so on. Most airline assessments have a short mental arithmetic test but then the rest is all about these other “competencies” - hands on flying skill is a consideration but airlines don’t need a fighter ace to fly their modern aircraft.

Equally, there is no point sitting next to Captain Einstein with a degree in Astro-Physics if the same Captain assumes he knows everything during an emergency, and doesn’t listen to his First Officer who may just happen to know something that he doesn’t.