The costumes were (mostly) judged based on craftsmanship and originality. When we were having our individual judging sessions, they asked a lot of questions about how each item was made, how long the costume took to make, etc. Making your own costume weighed heavily into it. So, I don't think it's that odd that one of the judges was a 'beginner' in the game. As long as she recognizes the costumes and the animations, then it's fine. Plus, even if she didn't recognize something right away, she could look up reference images to compare the costumes to the actual details.