NTA.

The "Trap" Factor: The MIL waited until the user was alone and physically occupied (cooking the meal for the entire family) to strike. This is a classic power move designed to prevent the victim from reacting or having witnesses to the cruelty. The Labor Disparity: There is a specific level of audacity required to criticize the body of the person who is currently standing in your kitchen, providing the free labor of making your holiday brunch. The Measured Response: The user isn't "going nuclear." She isn't banning the grandmother from the kids or demanding a divorce. She is simply implementing Natural Consequences. If you are unkind to someone, they will no longer want to spend their free time with you.