Mom says you are so ugly that I don't even know if you are my own. How to deal with it?

You can choose to respond or not to respond. If you choose to respond. It should not contain any content that directly derogates from parents. Words. You could say that. How can you scold yourself? It's okay if you don't want to respond, but ... I think in any case, you should know whether he has a deep meaning. Your mother's profound meaning may be that you have some pimples or pimples on your face, which makes him think you are ugly, or there are many scars, and so on. It is possible that he said this in the hope that you would change. It can be said to be a provocation.