Apparently calling someone a ‘ham’ has it’s origin in some politically incorrect history. The original phrase for an over-actor was ham-fatter but it has since been shortened.
Minstrel shows were old-timey shows with jokes and comedy (albeit racist comedy) involving white and black actors in black face make-up. These actors were considered lesser actors by their screen and theater counterparts. So a ham is someone who tries to overact, not understanding the subtleties of the art.
The main theories of origin are that these actors used ham fat to remove the black face make-up. The other origin story says it generated from the popular minstrel song “The Ham-Fat Man”. Anyone who would sing that song was an actor of poor taste.