Entering “Fashion Underground,” Susanne Bartsch’s Museum at FIT retrospective, is not unlike entering one of her famed parties. Visitors are greeted by a lineup of mannequin partygoers dressed to impress the “doorman” (equipped with an RSVP list) in a graffiti- and flier-laden alley—complete with an aluminum trash can. “It’s so New York, I should have had a rat run around somewhere, too,” says Bartsch ...