I think some people who have a good relationship with their mother don’t understand Kate. I have a good relationship with my mother, but I can certainly understand where her resentment and anger come from, considering how Rebecca is portrayed on the show.
I get it too. I am not liking Rebecca very much. Kate explained it well when she said her mom makes her feel like that insecure fat kid. You could see it in the way she was freaking out making the house perfect before Rebecca arrived. She feels vulnerable, imperfect, and ugly in Rebeccas presence. This brings out the worst in Kate. She lashes out. Especially the backhanded compliment after Kate sang at the bar. She said something like, "Honey, you did so great! In time you will learn how to work a big crowd." And there it was. The slap after the compliment.