Nirvana is not overrated in my opinion, they have many great hits that influenced all of grunge. They did it all in only 3 albums, which goes to show the talent they had. Smells like Teens Spirit is an overrated song, but is typically the only song "fans" know.
Bands like The Modern Lovers, The Jam, The aforementioned Clash, The Buzzococks, The Undertones and others all began at the same time as a nascent version of The Sex Pistols. Those bands also wrote much better songs and packed them with a much greater level of musicianship - which is to say some musicianship.
But it's their arrogant, annoying hardcore fans that make them one of the most overrated bands of all time. They go around everywhere saying that Zeppelin is the greatest band of all-time, with the greatest guitarist, singer, bassist, and drummer of all-time.