Etymology of "mullet"?
Etymonline says
The term in reference to the haircut seems to have emerged into pop culture with the Beastie Boys song "Mullet Head."
Wikipedia also mentions that source:
According to the Oxford English Dictionary, the term mullet was "apparently coined, and certainly popularized, by U.S. hip-hop group the Beastie Boys", who used "mullet" and "mullet head" as epithets in their 1994 song "Mullet Head".
but also mentions the 1967 film Cool Hand Luke and a 1982 episode of TV show Cheers as sources.
The carlobarbershop entry for 'Mullet' says:
The origin of the term "mullet" has been traced back to the film "Cool Hand Luke." Apparently one of the guys calls people with long, shaggy hair "Mulletheads."
A set of articles on squidoo.com mentions some of the same sources and then comments:
Until that point, mullet-head was simply an old-fashioned American insult denoting a stupid or foolish person. Around since at least the mid-19th century, mullet-head may derive from the synonymous British slur mull-head, or from mullet, the name of a widely consumed, flat-headed fish. The term was even used by Mark Twain in his novel Huckleberry Finn.
A Guardian page called Why is a mullet hairstyle called a mullet? includes some relevant reader comments:
Other names are slightly more self-explanatory: the hairstyle is also known as the "Kentucky Waterfall" or the "Mississippi Top Hat" in parts of the US, and the "Bouncing Cobra" in parts of West Wales. In Germany it is known as the "Vokuhila" (vorne kurz, hinten lang= short at front, long at back). -- Anna, Cardiff
and the claim
The hairstyle was first worn by French fashion guru Henri Mollet in the early seventies. The "Mollet" did not see much light apart from in the french underground dance scene, until it was ressurected by popular television personalities such as Pat Sharp, the word having been anglicised by this point to "Mullet". -- Gary Badger, Scarborough, England
and
Mr Badger, Please do not confuse our non-British friends by attributing the words 'popular' and 'personality' to Pat Sharp. -- Joel Bradley, London England
and
Because it looks like you have a dead fish on your head? -- Janet Edwards, London, England
and a comment from "Mare" in Destin FL who claims to have created and popularized the term in 1987.
Wikipedia has an article about it in English which partly answers your question.
French Wikipedia has a little more. It gives the name of this haircut in different languages with its sociological meaning and it says that in British English this haircut is so called because this haircut was popular mainly among fishermen. (The mullet is a common fish).
Anglais britannique : mullet = nom de poisson commun, apparemment dû au fait que cette coupe était portée principalement par des pêcheurs.
I suppose they are talking about English fishermen because in French this type of haircut is simply called "long in the back".
Mullock, meaning rubbish or waste, perhaps became mullet through slurs, language barriers, and general ill temperament towards those whom appear differently. Mullet does have an association in the United States with rubbish or trashy people. Citing phrases such as white trash used to describe some folks with mullets and other key trashy features. Hill billy, flat-lander, hick, redneck, white trash, trailer trash, poor folks, and scurrilous snollygusters are used somewhat interchangeably to describe those who are seen as less than the speaker. Mullet has gained an association with trashy for whatever reason.
----Disclaimer---- Those with mullets aren't anymore trashy than anyone else. Nowadays bleached hair and/or tips are widely considered far more trashy than mullets. Bleached hair, trashy make-up, indecent dress, and general a general lack of sophronization are often all tied to the same individual, however a challenge would be to find a mullet sporting, overall wearing, and animal butchering person whom EVER wears makeup.
Perhaps mullet's connotation in reference to hairdo should be replaced by "willwhoopyouruppiddyasset", "legit", or "Jeez, maybe I should take a look inward in order to understand why I judged someone based solely on a hairdo".
Henri mollet is where the word mullet came from but, the hair style is from The 6th century. Byzantine scholar (Procopius) males wore long in the back & short in front. Called a Hunnic.