My mother is a terrible cook, also. It was wonderful in the 50s, but not up to today's standards in terms of ingredients or recipes, full of fat and salt and sugar.
I do all the cooking now and she has never said much about it. She eats it, but that's about it. She used to help me wash up after, but not any more.
I started by bringing a dish or two to her house years ago, and it took awhile for everyone to start wanting my food more than hers. At first she was upset. After all, everyone used to love her greasy meatloaf with bacon, and pies made with canned filling. When they found out how good fresh food can be, they eased over to my cooking. My siblings and I were all excited about new foods, starting in the 80s, but Mom never got over her love of old fashioned food.
It took time, but she did get over it. I never bad-mouthed her food, just didn't eat it, and always brought alternatives. It was about new and different rather than anything against her.
No one ever got up in the morning wishing she'd eaten more the night before.
Original Goal: 114. Current old lady goal: 106.