Mona May had no  designs  on becoming a movie star.  But as the costume designer of cult classic “Clueless,” she covertly slayed a scene with Alicia Silverstone’s Cher Horowitz—  as if  being on camera were her true calling.  “It’s a really great-kept secret that I’m in the movie,” May exclusively told The Post of her blink-and-you’ll-miss-it cameo, proposed by director Amy Heckerling, as Cher’s massage therapist in the 1995 teen comedy.  “It was a last-minute thing,” laughed the style savant,