You do not need to lose weight. She is absolutely lying to you, probably in an attempt to make herself feel better about her own self-image. Thighs and arms and tummies need some skin in order to stretch properly, and 0% fat is not healthy, humans and animals need some fat on their bodies so they can function properly.
If she is very judgmental, even of other people, it may be helpful for you to point out in your head something you find is lovely about that person, like their fashion style or haircut. It should become easier for you to see that those people are lovely despite her judgments and hopefully when you look in the mirror you will find all those tiny lovely things about you too. I don't like lots of things about my body, but I like the shape of my lips, the strength in my legs, and my wrists. This could also help stop you from becoming a judgmental person like her.
Something else that may help is when you are feeling good about yourself, write down everything that makes you like your body and your image. This could include things like "I like the way this shirt fits my shoulders", "This blouse makes me look pretty", etc. If that doesn't help and you start second guessing yourself, go to a trusted individual like another family member or a friend and ask them to name a couple things they like about how you look. I love my friend's nose, the way she looks when she smiles, and her hands. You can fall back on these to be taken as truth because they come from someone who sees you in situations (when you laugh, see someone who makes you happy, eat ice cream, etc.) other than when you're standing in front of a mirror, which is about the only way we see ourselves. They also probably don't have the hangups your mother does that makes her try to tear you down.