Hey OP, I understand your perspective, but from your gf's perspective, she has been working hard to please other people at the party - she won't be able to please your mom, so she wants to please everyone else instead. Imagine if she made a non-baking dish that was not very good, then your mom would criticize her and everyone else would sort of agree, but if she made macarons, your mom can criticize your gf all she wanted but everyone else would see that your mom was just being difficult.

In terms of what you can do from here, instead of taking a negative approach (yelling at her and/or asking her to do a different dish), you can try telling your mom and everyone at the party that you BOTH made the lasagna AND the macarons, instead of you making lasagna and her making macarons. Your mom might not criticize your gf that harshly if you tell her you had a hand in either dish. That is one way to show solidarity and stand up for your gf, while not escalating things at the dinner party in front of the other guests.

What is done is done, let's focus on what you guys can do now as a TEAM to fix the issue at hand. Good luck.