Green’s Dictionary of Slang

mediocre, second-rate.

1894		Truth (Sydney) 1 Apr. 1/1: Wide-awake Americans [...] probably didn’t know which to smile at most — the Doctor or his hamfat exhibits.
1906		Sun. Times (Perth) 16 Sept. 1/1: A visiting ham-fat actor is on a wealthy wicket [and] he has kidded the rich relict to unbosom her pocket book.
1908		Chicago Trib. 29 Sept. in Fleming Unforgettable Season (1981) 267: Brush might subpoena a bunch of hamfat actors [...] and have the combat thrown out.
1910		Sun. Times (Perth) 30 Jan. 3rd sect. 1/8: That dreadful ham-fat actor and general behind-the-scenes-hog, George Willoughby, is approaching.
1914		Ade ‘The New Fable of the Father Who Jumped In’ in Ade’s Fables 91: Father had him kidded into believing that all the old ham-fat Riddles were simply Immense.
1918		‘Max Brand’ ‘Above the Law’ in Coll. Stories (1994) 30: Of all the tin-horn sports, the ham-fat, small-time actors, you’re the prize bonehead.
1959		S. Charters Country Blues 86: The singing of these little ‘hamfat’ bands never reached the artistic intensity of men like Blind Lemon.
1967		M. Williams Jazz Masters xvii: [T]hird- and fourth-rate players—’ham-fat musicians’ the better performers call them because they grease their instrument valves with ham fat.
1971		(con. 1900s) G.M. Foster Pops Foster 2: Uncle Wyatt was what we called a hamfat violin player—he wasn’t so good and he wasn’t so bad.
1973		Buerkle & Barker Bourbon Street Black 220: That’s where this term ‘ham-fat’ musician came from. It referred to some guys who, instead of takin’ lessons, just picked up their instruments and started playin’.