Green’s Dictionary of Slang

(US) a fool, a failure.

1966		Beverley Hillbillies [CBS-TV] He sure is a funny-lookin’ goomer.
1981		D.E. Miller Bk of Jargon 244: tennis ‘Goomer‘ [...] A loser, not only in fact and habit, but also by preference or temperament.
1991		B. Hamper Rivethead (1992) 73: It seemed Jim had a powerful soft spot in his lust-leechin’ heart for unemployed factory goomers.
2005		C. Adams Fear Ain’t All That 6: You’re the one who makes things happen...not some other goomer.