"hairstyle short on top and long in back," 1996, perhaps from mullet-head "stupid, dull person" (1857), also the name of a type of U.S. freshwater fish with a large, flat head (1866). 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 1275, thought to be a dim. of O.Fr. mul "mule."