For if you take apart 'Mamma, Mia!' ingredient by ingredient, you can only wince. It has a sitcom script about generations in conflict that might as well be called 'My Three Dads.' The matching acting, perky and italicized, often brings to mind the house style of 'The Brady Bunch.' The choreography is mostly stuff you could try, accident-free, in your own backyard. And the score consists entirely of songs made famous in the disco era by the Swedish pop group Abba, music that people seldom admit to having danced to, much less sung in their showers. Yet these elements have been combined, with alchemical magic, into the theatrical equivalent of comfort food.