Green’s Dictionary of Slang

mulletheaded adj.

[the freshwater fish the mullet has a notably large head]

stupid, foolish, ignorant.

Quindaro (KS) Chindowan 6 June 1: The men, for the most part sleepy, ignorant, mullet-headed looking wretches.
[US]Anderson Intelligencer (Anderson Court Ho., SC) 4 Mar. 2/2: Langley, a mullet-headed, muck-colored gentleman of African ’scent.
[US]‘Mark Twain’ Huckleberry Finn xxxix 370: They’re so confiding and mullet-headed they don’t take notice of nothing at all [DA].
[US]F.P. Dunne in Schaaf Mr Dooley’s Chicago (1977) 220: An old street car with th’ wheels off comes swimmin’ along undher command iv a mullet-headed Mayo man.
Ottumwa Tri-Weekly (IA) 27 Aug. 5/2: Ye lie, ye daytractin mullet-headed dayrogotory vituperator.
[US]Wash. Times (DC) 2 jan. 12/1: It was on this context that Merkle made the mullet-headed play that will be attached to him as long as he plays baseball.
[US]Alliance Herald (Box Butte Co., NE) 17 Feb. 2/1: It isn’t the mullet-headed girl who brings the dance into disrepute.
[US]Z.N. Hurston Jonah’s Gourd Vine (1995) 140: Well, you, ole mullet-headed tumble-bug, you!
[US]C.J. Lovell ‘The Background of Mark Twain’s Vocabulary’ in AS XXII:2 95: mullet-headed. Stupid.