"Fuck" is said randomly and arbitrarily without really having any meaning except to emphasise something. "Fuck, this is good" means that this is really good. "Fucking pirates" means that they are really annoying pirates.

I understand that there's a whole world where swearwords are a big no-no, but half of them are jokingly used between adults: as long as people know when or how to use them non-threateningly, I don't think it's a big deal.

"Dumb", on the other hand, is almost never used jokingly, though it can sometimes be used in a self-depreciative way.

Because "dumb" used to be a speech disability and then became an insult to intelligence which is offensive to those with speech disabilities.