People can be idiots for forming a firm belief that something can be bad for them when all evidence and expert opinion contradicts it.

Anybody, idiot or otherwise can, due to the nocebo effect, trully feel bad due to nothing else than a strongly held belif that something will make them feel bad.

Those are two separate things.

There aren't indications of idiocy from feeling actual effects due to nothing else than a placebo or nocebo effect from a strongly held belief, whilst there can be idiocy in the way one gains a belief so strong was that it actually triggers one of those effects.

The example given by the previous poster does seem to fit a situation of people who are indeed idiots not because of how they feel but because of how they came to believe they would be feeling like that in a certain situation.