ISAAC, ON STAGE: Isaac Mizrahi let it all hang out Thursday night at the West Bank Cafe on 42nd Street, where he opened his cabaret act called “Ask Isaac.” Anybody wondering whether Mizrahi’s stints at Target or QVC were just stepping stones to the nightclub circuit need not worry. But aside from a few bumps into the music stands and fidgeting in a tight black jacket, Mizrahi kept his cool, had fun performing, and even hit the high notes without over-reaching. He sang a medley of popular tunes ranging from “C’est Si Bon” to “I Believe in You” and kept in his zone, which involved a lot of bantering back and forth with the audience, and the band — “Would you believe they’re all straight?” Mizrahi philosophized on his favorite things — Xanax and physical therapy which he didn’t expect a regular diet of at his age. “I recently turned 50,” Mizrahi said. “More recently, I turned 51.” Getting married, he said, was like having kids or enlisting — it never really occurred to him. He recently tied the knot anyway. “I wanted to fit into a demographic,” Mizrahi said. Asked what he thought the biggest fashion faux pas would be, he said simply, lip-liner.