What sounded Special Snowflake to me was that she expressed to you that she wanted to be able to attend your parties, that she was sorry to have missed the fun of last year's party and that she would love to attend this year's party if only you would call her to tell her that no one had shown up in a mask.
I feel that she should have kept her angst to herself and not put that on you. She should just have a personal policy of no costume parties. It's good that she didn't ask you to ban masks at your party. But I don't think she should have let you get stressed about it, either.
But I'm really glad to hear that a good time was had by all! I'm not a doctor but I can't help but wonder if a medication before an occasion like this might be something that she'd want to try.