I agree that stupid might not be the right word. He is lazy, ignorant, and incurious doesn’t he often says stupid things but the source of it isn’t necessarily a lack of intellectual capacity it’s that his mental illness prevents him from taking in and processing information normally. That’s all different when it comes to feeding his narcissism though.
So yes, if you look at normal things like running a company or running the government or whatever for all intents and purposes he’s extremely stupid. He’s been a failure at both despite massive support and advantages because he’s incapable of thinking problems through and planning long term. When it comes to things that can provide him narcissistic supply though like public relations, displays of dominance, fighting with people, etc. those things have his complete attention and he’s quite good at them. The capacity is there, it’s just functionally inaccessible for anything useful.
Also at this point it seems like he’s suffering from serious cognitive decline so whatever he once had is getting worse. His brain appears to be basically pudding at this point.