The references have everything to do with the way Grey Poupon was marketed as a upper-echelon condiment; a mustard for the rich and high society. That's why we hear rappers either brag about using it, or in 50 Cent's case, rag on Jay-Z for being a "pass the Grey Poupon ass nixxa..." It can be used in a braggadocios manner or as an insult to someone who has abandoned their hood roots. Also, "Poupon" just sounds like something expensive lol.