I think some of you are confusing over/underrated guitarists with good/bad guitarists. Someone's ability to play guitar does not have anything to do with his ability to write music.
The Beatles will never be remembered for their musical talent, but their mark in music history is deep because they knew that music doesn't have to be complicated to be beautiful or revolutionary. Bob Dylan is remembered because his music was great, in spite of a below average voice.
You can't fairly judge someone on his guitar playing ability when he makes no claims to be good. Also, people who are calling Kurt Cobain overrated--who has ever said he was an expert guitarist? If somebody thinks he is a great guitarist, please elaborate.
That said, I think Ace Frehley is a good guitarist, but he has to be considered overrated, because
he couldn't perform some of his studio solos live. This doesn't make him a bad guitarist, but it does make the perception of him overrated if anything. An
underrated guitarist wouldn't be limited by his own band's playlist.