“Fluffy as a pompom and savvy as an A.V.-club nerd, the new musical “But I’m a Cheerleader” would be just another oversold confectionary treat without its big, beating heart. In their adaptation of the sweet-spirited if self-consciously quirky 1999 film, the librettist and lyricist Bill Augustin and the composer Andrew Abrams have improved on the original by taking its ironies seriously and giving them a bouncing backbeat. The production, directed by Daniel Goldstein and choreographed by Wendy Seyb, looks well on its way to Broadway polish.”