There's a fine line between tacky and spectacular. In creating costumes for Cher over the years - costumes that often tell the story of a shy woman emerging triumphant from a chrysalis - the designer Bob Mackie has kept on the right side of the line by making sure the level of craft supports the extravagance of the gesture. Sadly that's not the case with 'The Cher Show,' the maddening mishmash of a new musical that opened on Monday at the Neil Simon Theater. Except for the dozens of eye-popping outfits Mr. Mackie gorgeously recreates for the occasion, it's all gesture, no craft: dramatically threadbare and surprisingly unrevealing.