Non Sequitur Scene: TWO!!!! Dancing Queen starts decently as the Dynamos are trying to cheer up Donna, but ends with all the women running off to join in a random dance by the pier (that's never mentioned again). And then the ending where everyone gets dumped with millions of gallons of water. Sure it's been set up as a Brick Joke, but it barely lasts a minute and seems to be an excuse for Fanservice
Meryl Streep actually won an award for her rendition of Mamma Mia.
Ending Fatigue: There are three musical numbers after the plot is wrapped up, plus the first portion of the end credits has two more songs. Amusingly, this opened in the U.S. the same day as The Dark Knight, which also had trouble wrapping up.
The stage version ends this way as well, but "Take a Chance On Me" occurs before the wedding, and "When All is Said and Done" doesn't occur at all.
Family-Unfriendly Aesop: Sleep with three guys and not know your baby's father? No, don't take a paternity test, wait a few years and have fun singing about it.
Germans Love David Hasselhoff: Due to premiering on the same day as The Dark Knight it wasn't as big a success as it should have been. However, it did extremely well in Europe (where superhero movies aren't that popular), and is the second highest grossing movie of all time in Great Britain.
Actually, releasing it on the same day as The Dark Knight (as counterprogramming) was an intentional choice, and may have even helped its success. It made $144 million in the US, which is huge for a musical chick flick. Moulin Rouge, for example, was considered a success at $57 million. Of course, its ridiculous foreign gross still means that its an example of this trope.
Not to mention it actually did outgross Avatar in ABBA's native country, Sweden.
Narm Charm: The ladies in full getup after the movie's fully over? Alright, you can take that with a giggle or two. But when the GUYS come out in costume for "Waterloo"? Oh dear God.
She Really Can Sing: Amanda Seyfried surprised a lot of people. In fact, one of the reasons she was cast was because she could pull off a high note during "I Have a Dream" ("I believe in angels...").
Tear Jerker: Two moments right in succession: Donna singing Slipping Through My Fingers while getting Sophie ready for the wedding and The Winner Takes It All sung by Donna to Sam a few moments later.
Also, one critic commented once on how she keeps touching her face and using her expressions mostly for acting, which makes it seem as if she’s just a head. One journalist in Israel asked her if she was trying to prove she has a body as well, and Streep replied this might have been the case.
Better example would be Pierce Brosnan. Clearly he was cast to be swooned over rather than listened to.
According to him, the casting directors didn't know he couldn't sing, and he didn't know it was a musical, until the paperwork was signed. All he cared about was finally getting to work with the girl he had a crush on back in acting school.