"hairstyle short the sides and long in back," 1996, perhaps from mullet-head "stupid, dull person" (1857). Mullet-head also was a name of a type of North American freshwater fish with a large, flat head (1866) with a reputation for stupidity. The term in reference to the haircut seems to have emerged into pop culture with the Beastie Boys song "Mullet Head."
#1 on the side and don't touch the back
#6 on the top and don't cut it wack, Jack
[Beastie Boys, "Mullet Head"]
As a surname, Mullet is attested from late 13c., thought to be a diminutive of Old French mul "mule." Compare also mallet-headed, in reference to the flat tops of chisels meant to be struck with a mallet.