Naw osu! Is all about neuro motor stuff. Aka not related to your cerebral cortex but coordination and muscle memory.
I'm actually interested in people that are especially bad at osu!, but I think it can usually be explained by lack of proper practice. If you are trying to get better, there are lots of ways to waste time in the game by playing the "wrong" maps. And even if you are playing the right maps for your improvement, your quality and kind of focus and motivation also play a large factor.
So basically, there are a lot of reasons someone could be trash. But none of them probable have to do with you being stupid.
There is an argument to be made that a dumb person would have trouble identifying the right maps to practice on, and I think if you have severe mental disabilities rhythm and musical sense may be hindered (citation needed). But even then, someone could just tell you what to work on and you could memorize that or play songs out of a list. And if you really didn't have any musical ability you could still learn the game through pure visuals.
There is a type of mentally disabled that was once called "custodial retarded", which is now a very dated and offensive term by most standards. These kinds of people would have problems with basic tasks like tying their shoes, dressing, and feeding themselves. These are probably the only category of disabled people I could see not having the basic understanding or coordination to become good at this game. But there's no way a person like this could even make your forum post.
So if someone has a lot of hours and is still not improving, I would guess it's because they have some sort of aversion to practicing properly, or have some sort of coordination disability, of which I know nothing about but probably exist.