Standing against a Greek-island sunset in Mamma Mia!, Meryl Streep's single-mom character, Donna, declares she's a loser at romance: ''No more ace to play,'' she sings ruefully. True for Donna, perhaps, but at 59, Streep is throwing down aces all over the place. Last summer, Mamma Mia! raked in $559 million globally a girl-power milestone. In December's Doubt, she banishes all frivolity as pop-music-bashing Sister Aloysius, a Catholic-school principal convinced that a priest has sexually abused a student. While Oscar buzz around her role is louder than those ABBA songs, Streep wishes adult dramas weren't always year-end-only, awards-bait affairs. ''I just wish sometimes that we'd have more stuff like Doubt in the summer,'' she laments. ''But that's against the laws of Hollywood.'' You said it, sister.