The question is what famous guitarists are mediocre players?
There are a MANY famous musicians/individuals who aren’t great at playing guitar, and got their fame because they excel in areas OTHER than playing guitar, including singing and composition.
I’ll name one as an example.
Elvis Presley played guitar, and performed with it. He could accompany himself with basic chords, and he could play some basic blues things. He could also play piano.
As a guitarist, he had modest chops, but he was a fantastic vocalist and performer, and that’s what made him famous, not actually playing guitar. He also worked in a band with other great guitarists so he didn’t need to have those chops.
If the question is which guitarists who got famous specifically for playing guitar who weren’t actually that good. . .that’s a much harder one to answer. I don’t know any guitarists with a reputation for being great players that are actually BAD at it. Being a pro guitarist is a tough and competitive business, and people who earn reputations for playing generally deserve it.
There are a number who might be considered “overrated”, in the sense that their reputation as guitarists probably exceeds their actual technical and musical abilities, but that’s not to say they’re not “good”.