Green’s Dictionary of Slang

(US) foolish, stupid.

1855		Life in Boston & N.Y. (Boston, MA) 14 Apr. n.p.: A cotton-headed pimp by the name of Ed L—s.
1860		Wkly Standard (Raleigh, NC) 1 Aug. 2/2: A lazy, lolling, tallow-faced, cotton-headed, lack-lustre-eyed boy [...] the personification of stupidity itself.
1913		University Missourian (Columbia, MO) 13 Oct. 3/4: A cotton-headed lawyes speaks to his friend, a shrimp journalist. ‘Hi you shrimp journalist’ [...] ‘Hello, yourself, you little cotton-headed freshman lawyer’.
1937		E. Anderson Thieves Like Us (1999) 2: Come on, you cotton-headed old soldier!