The Sunday Mirror obviously agrees with you as to the 'dodginess' of the mullet. Here's a quote from a 1998 edition, "The mullet...if you have one, then a word in your ear: Scissors."
In American English, a mullethead is a flat-headed fish and here's a quote from there in an edition of Harper's Magazine published in the 1860s about it, "Dat fish is a mullet-head; it hain't got any brains."
Seems the idea of daftness regarding the mullet exists on either side of the Atlantic!