You can't create art about the mafia---in literature or on film---without showing that that the characters are of Italian descent. While it's true that less than 1/10th of one percent of Italians in America are in the mafia, the mafia remains 100 percent Italian.
If by "the mafia" you mean made members, then yes (although I've read that some families--at least at some point--only required that someone have an Italian father in order to become a made member). But it should also be noted that there are many more associates than made members, and they do not have to be Italian. In fact, a number of very prominent associates have not been, such as Jimmy "The Gent" Burke (who was affiliated with both the Lucchese and Columbo families) and Gus Alex (who was an important member of the Chicago Outfit).
