Attending the opening night performance of Wrong Moutain are filmdom's Barbara Barrie and Oscar winner Olympia Dukakis. At right, husband and wife acting team Phoebe Cates and Kevin Kline. Wrong Moutain, an often savage look at the American theatre, concerns a bitter middle-aged poet who bets his ex-wife's new husband, a wildly successful Broadway playwright, he can write a smash play - and does. Mountain, by David Hirson of La Bete and starring Ron Rifkin, performs at the Eugene O'Neill Theatre.