Green’s Dictionary of Slang

dim bulb n.

(US/Can.) a fool, a dullard.

1927		T.A. Dorgan in Zwilling TAD Lex. (1993) 31: Oh she’s dead from the neck up — A dim bulb — She thinks Pearl Harbor is a night club hostess.
1930		‘Leslie Charteris’ Enter the Saint 98: I’m afraid that’ll make Betty out to be rather a dim bulb, but we can’t help that.
1930		E. Glyn Flirt and Flapper 108: Flapper: Man isn’t vile [...] he’s only a dim bulb.
1997		G.V. Higgins Change of Gravity [ebook] When I go, what goes will be me. Me, pissin’ and moanin’ like always, not some dim bulb that finally burned out.
2001		S. King Dreamcatcher 81: At Gosselin’s, dimbulb!
2004		Mad mag. Apr. 37: America’s brightest minds and dimmest bulbs travel across country.
2012		T. Piccirilli Last Kind Words 227: Butch was a dim bulb [...] I wasn’t going to be able to talk any sense into him.
2023		(con. 1962) J. Ellroy Enchanters 389: The grasping ‘dim bulb’ Marilyn [Monroe].

In phrases

have a dim bulb (v.)

to lack intelligence.

1977

C. McFadden Serial 100: I used to think she had a pretty dim bulb myself.