After a spotty rendition of “Ain’t Misbehavin’” to open its season, Theatre By The Sea has moved on to the musical that will not die, ”Mamma Mia!” And it’s a big step up from “Misbehavin,’” but still not without its shortcomings.Malia Monk, fresh out of Boston Conservatory, was way too green to carry the lead role of Sophie, who goes in search for the father she never knew to walk her down the aisle.