What I am saying is that evidence of stupidity should lead you to raise your estimates of the probability that the speaker is stupid.

...in the context during which they exhibited the behavior which generated said evidence, of course. In broader contexts, or other contexts? To a much lesser extent, and not (usually) strongly in the strong-prior sense, but again, yes. That you should always be capable of considering further evidence is - I am glad to say - so universally accepted a proposition in this forum that I do not bother to enunciate it, but I take no issue with drawing conclusions from a sufficient body of evidence.

Come to think, you might be amused by this fictional dialogue about a mendacious former politician, illustrating the ridiculousness of conflating "never assume that someone is arguing in bad faith" and "never assert that someone is arguing in bad faith". (The author also posted a sequel, if you enjoy the first.)

As an aside, I treat how a person argues as a much better indicator of stupidity than what he argues. YMMV, of course.

I'm afraid that I would have about as much luck barking like a duck as enunciating how I evaluate the intelligence (or reasonableness, or honesty, or...) of those I converse with. YMMV, indeed.