I think the term "overrated" gets thrown around too loosely. For example:
KISS were not great musicians, nor songwriters. They were a party rock band known for their gimmick and showmanship. In that context, they seem pretty fairly rated. No one is calling them Stevie Wonder level musicians or songwriters.
Nirvana was a basic three-piece grunge band. They had a very dissonant sound, very power chord heavy, interesting lyrically, and Cobain's voice, while not technically greet, was unique. The sound worked for the Gen X, flannel-wearing teens of the time. Not overrated in that context. Adding higher-level musicianship might have actually been a detriment to their sound.