It’s not easy to be a beauty and a funny girl — a va-va-voom body tends to distract from the jokes. But Katherine Heigl has the knack — look at Knocked Up. Just don’t look here. Heigl fights an uphill battle in 27 Dresses, a chick-flick compendium of wedding cliches that will have every guy bolting for the exits. Screenwriter Aline Brosh McKenna also wrote The Devil Wears Prada, but she’s dipped her sharp wit in sentimental syrup. It helps to have Judy Greer around as Heigl’s acid-tongued BFF. But McKenna and director Anne Fletcher (Step Up) make a fatal error. They want us to believe that the gorgeous Heigl is a plain Jane who saves her bridesmaid dresses — all twenty-seven of them — while stoking her unrequited love for her macho boss (Ed Burns). Not buying it. Even James Marsden, so good in Enchanted and Hairspray, can’t put any zip into the role of a cynical wedding columnist who morphs unpersuasively into our girl’s Prince Charming. 27 Dresses is so flimsy it gives froth a bad name.