Well, on the one hand, you had two funny-looking guys with bad haircuts, and (especially at the beginning) all four of them in some of the goofiest costumes to have ever appeared on stage or television.
On the other hand, you had two attractive women dancing side by side in costumes that were frequently skin-tight, or fairly skimpy, or both.
In between those two hands, I don’t think many people were thinking much about teeth.
(The perfect teeth thing is really quite modern and to a certain degree American anyway.)
Mamma Mia isn’t my favorite ABBA song, but this one is. This is the last live performance and it’s sad to watch them and know what they’ve been through.
Like them or not, ABBA was a major mover in the music world and they were very talented folks.
The Winner Takes it All is a really good song, especially given the circumstances within the group at the time. They had a lot more going on there than a lot of pop bands do.
The kind of dental care that is common now even for the average person, was not available even for celebrities back then. Invisible braces? Forget it! Most of the teeth straightening devices of the time looked like the aftermath of barbed wire war. Metal grills with wire through them that got tightened periodically. My girlfriend’s even had rubber bands that clipped on the inside for tension. She could barely talk, singing would have been impossible.
That and the value/importance of straight, white teeth in order to succeed as a celebrity was not there either. No MTV, or internet. And not nearly the papparazzi coverage 24/7. People would see a singer perform in person or on TV occationally. The rest was just heard on the radio or album (vinyl).
Singers, especially before the day of the music videos were less apt to fix their teeth because it effects the way they sing. I had a front tooth capped, and it looked find but then I started to lisp, and it took me two tries for them to cap it so I didn’t sound like Cindy Brady.
I can imagine if you sing it’s even harder, so singers probably are weary about getting teeth altered if at all possible not to.
Also back then it was common for actors to wear temporary fake teeth. I read how Bette Davis, Lucille Ball and Dawn Wells (Mary Ann Gilligan’s Island), all told stories of how they had to wear caps over their teeth for their filming then when the filming was done they’d take these false caps off.
So back then even when they altered the teeth it was easy to forget to put them on because they didn’t permenently alter them. So she may have worn fake caps for a photo session and took them off when she sang