IT’S EASY to get cynical about celebrity collaborations. But considering that singer Demi Lovato devoutly practices mixed martial arts—and even met Kate Hudson, the actress who founded exercise-gear label Fabletics, in a gym—Ms. Lovato’s new collection for the brand passes muster. “Demi felt like a perfect fit,” said Ms. Hudson last week at the Beverly Hills Hotel, where friends like Amanda Peet and Molly Sims had gathered to celebrate Ms. Lovato’s designs.

Among the guests, there was not a legging in sight. Instead,…