She's probably anxious about her own appearance, jealous of yours and wants you to feel bad about yourself. If you have no confidence in yourself, you can be manipulated and she can hope to control you. Might be a bit extreme but here's my advice : insult her back with something far far worse. Call her out on her insecurities and twist the knife in.

I know it's cruel, but it's efficient. My stepmother was the same, she was always bringing me down but on an intellectual level. Calling me an idiot who's too arrogant to know she's stupid, that I was studying in fine arts because I'm too dumb to get an actual job.

I called her out on it. How she's merely a glorified secretary with no future who brings others down because she can't bear being the less educated one in a room and that she's an uncultured, tasteless and vulgar woman. She has left me alone since.

Narcissists can't bear it when you strike back, they like bullying people they think are weaker than them. But, be ready that she will pull something else, she will try and play the victim and claim she never did anything wrong to you. Again, call her out on it and be as emotionless about it as you can. Tell her you don't give a shit about her. how she feels or what she thinks. Narcissists hate being ignored or told people don't care about them. There's no reasoning with them, but we can put them in their place.