It's a word used by some young Italian migrants I've met in London. It means stupid, loser; someone who doesn't follow fashion and therefore is not stylish at all. The word "melotta" can also be referred to an awkward behaviour. I was told that the word "melotta" derives from a very low price brand of whiskey, in Abruzzi, a region of South Italy.
Look how he's dressed! He's a real melotta!
Don't be a melotta!
What a melotta he is!
You really are a melotta...