YTA for you and your mom too.

My mum is pretty serious about hosting dinners too and spends a lot of time planning menus and making sure our guests have a lovely time. She would never even think of hosting a "tasting" competition to reject people's offered contributions, no matter how great or not-so-great of a cook they are. At first I thought you meant it was to make sure everyone brings something different and everything paired well, which would still be a lot, but this is just so far beyond ridiculous. Over CHRSTIMAS COOKIES?? It really does seem like a thinly veiled attempt to deliberately exclude her. The fact you apparently can't see that makes me feel sorry for your wife, who is clearly still trying hard to help out, just to be met with this bonkers level of hostility from your mom.

If your mom cared about her guests' experience, she would actually consider your wife's feelings too. I wouldn't want to go to that Christmas party either. It's worse that she tried to speak to you about it only for you to both brush off your mother's actual crazy behaviour, and then dismiss your wife as being the "unreasonable" one. What's unreasonable is expecting her to participate in this asinine tradition of your mother bullying her guests.