You were right to shush the person singing along to Mamma Mia. There really is no need to do so, there are usually backing vocals on stage already that make the lyrics hard to understand without any extra distractions.
No one should feel they HAVE to stand up for an ovation. I have remained sitting when others around me were standing. On the converse side, I have also given a standing ovation to a show's ensemble.
I second the latecomer's pet peeve and people who talk during the show, but I also am annoyed by people who do not seem to care that they are in a theater, whether it be by how they dress or how they act, or who try to talk about the show at intermission and are misinformed.
Dean: Can I tell you something?
Lorraine: That depends on what it is.
Dean: I think you're really really pretty.
Lorraine: (after a pause) Ok, you can tell me that.