Pinkie is being Pinkie. People claim character development in this show, but she's the same pony that totally annoyed Rainbow and Gilda, Cranky, and pretty much everypony else at some point or other. And in that sense, it's just fine because Pinkie Pie being nuts is part of the show's raison d'etre.
As a big a cappella fan, this was a real treat for me. I really liked that Rarity was NOT the vocal lead and hogging the spotlight, though as the Soprano she couldn't be missed. Great use of continuity with the poison joke, though Fluttershy has been on stage in front of people as part of an ensemble, or as a model many times. As such suddenly being unable to sing before a crowd is weird, but still fitting with her character.
In fairness, Fluttershy really is trying to get better at being more assertive, but her very nature is that of someone who is willing to be pushed into the background to help someone else. The problem here is that she has to be in the foreground, and that's what scares her.
I love a capella myself, and was in a barbershop quartet when I was in high school, and it was so much fun, and I didn't mind being out in front to be seen, because I was used to performing. Still, singing leaves us vulnerable, because it's something we do out of joy. Opening oneself to criticism puts that joy in danger, and I think that's what truly scared her.