I'm slightly mistrustful of titles that include exclamation points. They always remind me of the musical version of The Elephant Man, Elephant!, in The Tall Guy ("... there's an angel with big eeears..."). But in the case of Mamma Mia!, I'm actually surprised the title only included one exclamation point -- you can imagine the filmmakers or the creators of the stage version embracing even more emphatic punctuation, just to let you know that This! Is a Musical! And also Wacky!! As if chorus lines of men in flippers, Meryl Streep waving a feather boa, and enough ABBA music to sate the leads of Priscilla, Queen of the Desert wouldn't have clued you in.
The movie, like the stage musical it's adapted from, is essentially and unabashedly an extended gimmick -- an excuse to sing and perform songs that originated from the Swedish musical group ABBA. Characters spontaneously burst into song not because they're aspiring performers (Chicago), or because their singing is meant as a melodious soliloquy (Sweeney Todd), but because the situation or their emotional state reminds them of an ABBA song (sometimes more tangentially than others), and they decide to share it with everyone. I've had friends like this in real life, although that seems to have been a college-age thing.