It's a brave man who has his hair styled in a mullet! The definition in Chambers Dictionary is "a hairstyle that is short at the front, long at the back, and ridiculous all round" and a couple of the citations in the OED indicate that it is most certainly considered ridiculous: "Connoisseurs of the absurd in gentlemen's hairdressing have been enjoying a laugh at..those unfortunates who continue to commit that most heinous fashion don't, the Mullet", and "The Mullet... If you have one, then a word in your ear: Scissors".
Quite why the hairstyle is called a mullet is uncertain. It might be named after the fish, according to some sources, or perhaps after the obsolete verb to mullet, meaning 'to pluck with tweezers'. The likeliest origin is perhaps the slang word mullet-head, which originated in the USA in the 19th century and means, according to the OED, 'a stupid person' or 'an imaginary fish having no brains.'
I was prompted to write about the mullet because I learnt the German word for it recently (thanks, Lizzie). It's the lovely word Vokuhila which stands for vorne-kurz-hinten-lang, or 'front-short-back-long'. Picture and more info (in German) here.