At first sight, it might not be clear exactly what is wrong with the arguments for that conclusion, but I've got a feeling that something is wrong with the arguments. However, I can't assert the conclusion is false unless some tests are performed on the arguments. Well, we begin by taking a classical method found in Aristotle’s Prior Analytics called Reductio ad Absurdum, which means Reduction to Absurdity.

EXAMPLE (1): Now suppose that α is an all-round Physicist, except that he has little interest in Theoretical Physics, and thus he lacks knowledge in that branch of physics, then would it be logical to say α would ever stupid in that sense?

EXAMPLE (2): So when β lacks mental processing power or the ability to understand Einstein Field Equations (EFE) in Albert Einstein's General Theory of Relativity, would it be probably fair to say that Stupidity criteria made β against itself?

EXAMPLE (3): κ is generally good in Applied Physics, but when it comes to Theoretical Physics, everyone tends to think κ takes a long time to study, and he still doesn't grasp the subject no matter how many materials he studies, then would everyone ever think κ is a stupid person? (Einstein's stupendous effort on the General Theory of Relativity is a measure of the difficulty of the problem that it took even him as the one of the World's Greatest Scientist approximately 10 years (1907-1915) to fully understand how to do this.)

But it is really ABSURD to say “Yes” for that three examples. And this means that α, β and κ, cannot be judged stupid, contrary to the hypothetical arguments that they are. Accordingly, since that initial commensurability assumptions engendered contradictions, I have no alternative, but to reject that three initial conclusions.

Nevertheless, in my opinion, although the reductio tests do refuted the original forms of the arguments, but the main force of the arguments can be merged and restored by a minor amendment. Perhaps, most people would say someone is stupid, when observing someone's seemingly action is lacking common sense or showing poor judgment or making repeated mistakes in a subjective context, without suspending judgement.

Sure you can have your own opinion of what stupidity means to you. But opinion is not the same thing as truth. Let say El Nino effect may seem blazingly hot to you, but just mildly hot to me. Does that mean there are two different truths about the temperature 35 °C, yours and mine? Of course not. It just means that we experience things, even the same things, differently.

See EXAMPLE (1) above. By the way, the judgement of stupidity is based on the perspectives of others POVs on the levels of Common Sense, not personal POVs.

Maybe you borrowed that saying from Aristophanes quote. I'm wondering how did you casually / seriously characterize that at the PhD level, though it maybe true for your experience if you are forever on that belief. Perhaps there is common ground we can find from different cultural backgrounds. Can you ascertain further than that belief by relating to some quantitative & qualitative findings, so others could share those values?