“Fashion Underground: The World of Susanne Bartsch,” which debuts today at the Museum at FIT in Midtown Manhattan, focuses not on a designer or thematic fashion idea, but on a woman who embodies the idea of the fashion muse. Curated by Valerie Steele, the exhibition takes viewers on a whirlwind tour of Susanne Bartsch’s 30-year career as a fashion renegade, avant-garde boutique owner and spectacular party-thrower. These extravagant parties, filled with gyrating bodies in outlandish clothes, were nightly spectacles of performance art. Hosted at various venues, such as Savage, Copacabana and Le Bain, the parties attracted the fringes of the fashion world—“the people who are obsessed with fashion who might not be professionals,” said Ms. Steele ...