Among the late '50s Famous cartoons written by Irv Spector, "Finnegan's Flea" isn't the most disturbing -- not while "Chew Chew Baby" exists. But it's still very disturbing, like "One Froggy Evening" except with Paramount-owned songs, a protagonist whose life is shattered through no fault of his own, and a truly unpleasant image to start and end the film. See also Rachel Newstead's long analysis of this cartoon, from 2007.
The thing is, though, that the big twist in this cartoon is an old one. I'm pretty sure I saw it in sketches/stories that pre-date this one, though usually it was the talent agent or producer who did the deed. ("Ugh! A bug!" WHAM! "Now, what was this great new act you were going to tell me about?") Leave it to the Famous people to take an old joke and play it for tragedy and living death.