To clear up the obvious question, “Bad Cinderella,” which opened at the Imperial Theater Thursday night, isn’t good. Composed by Webber and with lyrics by David Zippel, it is a muddled and momentum-less retooling of the familiar fairy tale in search of a coherent point of view as if it were a glass-slippered foot. The book, originally written by Emerald Fennell, the Oscar winning screenwriter of “Promising Young Woman,” and adapted for Broadway by the playwright Alexis Scheer, is an illogical head-scratcher, despite being based on a story most everyone knows. “Bad Cinderella,” directed here by Laurence Connor (“School of Rock”), even manages to gleefully reinforce the chronic social fixations — on beauty, vanity and wealth — that it purports to deem toxic.