A bottom-feeder is, first, a fish that eats off the mud at the bottom of a river or the sea. People took that picture and used it, with a sour face, for a person who makes money when other people are in trouble — a company that fails, a family forced to sell cheap, a scandal. The bottom-feeder moves in and profits. It is informal, and it is never a compliment; you say it to show you disapprove. Newspapers and office talk use it more than careful essays. It is about feeding on someone else’s lowest moment, not about being poor yourself, and not about holding a low-status job.