With his unmeltable snowcap of hair, Steve Martin has looked exactly the same on screen for almost 20 years, but in Cheaper by the Dozen 2, the fractures are starting to show. Returning for another celebration of cheesy dadness as the overzealous father of 12, Martin is hiding under so much creamy, age-defying face cake that he looks ready to fill in on a talk show. It underlines everything wrong with this very poor comedy, a family fantasy that plays at being happy and picture-perfect but primarily comes off as fake and dismayingly unfunny. ''Life's blazing by!'' Martin cries at the outset to his brood. ''Let's go up to the lake one last time.'' There's your entire premise. Set up Eugene Levy in a gigantic cabin across the water as Martin's foil in competitive parenting, and what you have is less a sequel to a not-so-bad remake than yet another remake, this one of that not-so-great 1988 John Candy comedy The Great Outdoors.