Mullethead is a slur used to describe Whites in a derogatory way. Bad hair cut (short on top, long in back) rural whites sport fashionably. Could easily be shortened to just "mullet."
The word Mullethead is a racial slur that has been used historically to degrade and dehumanize whites. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to whites. If you are unsure about whether or not it is appropriate to use the word Mullethead, it is best to err on the side of caution and avoid using it in any situation.