If it was still up in the air whether Fred Armisen and Carrie Brownstein are laughing with — instead of at — well-meaning but oblivious hipsters on Portlandia, the question was answered once in for all on Tuesday night at Bowery Ballroom in New York City. There, during a solo set, Armisen was joined on-stage by Marnie Stern, Tim Harrington of Les Savy Fav, and Leigh Nash of Sixpence None the Richer for a cover of TLC’s “Waterfalls.” Nash assumed the roles of Chilli, T-Boz, and Left Eye. If this sounds like a sketch — a bunch of indie rockers in matching white jumpsuits faithfully strumming out a twangy cover of one of the biggest R&B songs of the ’90s with the crowd joining in for a choral breakdown at the seven-minute mark — it’s because it almost literally is. The punchline, presumably, is the mere existence of the act. What better way to pump your show with credibility than to embody its entire joke?