I find your comment interesting because you are implying that some people believe being stupid or clever is a permanent unchangeable state. Presumably one is born as either one or the other?
I would say that some ways of thinking are stupid. In particular when one does not challenge one's assumptions. It's possible to build a whole world of stupid on top of bad assumptions. If someone's entire worldview is built in this way - a whole load of bad assumptions held together with poor logic and wishful thinking - I don't think they're even living in the real world any more, they're living in a fantasy land.
.. just don't tell them it was with yourself