I've always wondered if Debra isn't a decent cook. She's just not of Marie's caliber.
I think Marie was determined to let everyone know that she did not think Debra was a good cook. I don't think it mattered if Debra was good or bad to Marie - she was set on telling Debra and everyone else how bad she was.
The episode where she switched the contents in the spice jar to make Debra's meatballs taste wrong is a good example. She told Raymond she did it so that Debra would try and fail. It is pretty sad that Marie's goal was to make her daughter-in-law fail as a cook just to make herself feel superior. Various times she was talking about cooking and she would say to Debra something like 'Oh, I'm sorry but you would not understand - you have to be a cook to know what I am talking about."
Although Marie did not seem to dislike Amy as much as Debra, she still had to act like her food was better than Amys. In the episode where Frank and Marie go to Amy's for brunch (and meet her parents) - Amy has everything spread out on a table. Marie walks in with something she decided to bring (unasked, of course) and told Amy to remove something of hers to make way for Marie to put her food on the table.
Debra and Amy were sure not perfect. But I think they were both unbelievable in putting up with Marie for a mother-in-law. In fact seeing how Marie treated Debra, it is almost surprising that Amy would want to marry into that group.