You may be a tad moonstruck when you see the poster for the Twentieth Century Fox comedy Used People, starring Shirley MacLaine. After all, it's got the same backdrop a New York City skyline and a big ol' moon as the poster from 1987's Moonstruck, which featured a bedazzled Cher. But Richard Ingber, president of marketing for Used People's production company, Largo Entertainment, insists that ''the design had nothing to do with Moonstruck, though the moon does have a part in this movie.'' Ingber adds that he has never heard the two movies compared. If so, he hasn't been reading Used People's recent ads, which prominently feature a critic's quote: ''As charming as Moonstruck...'' Must be the lunar pull.