If a mother tells her daughter to do something and that something is NOT against halacha, the daughter is obligated to do as her mother asks,out of Kibud Av v’Eim. The Torah specifies the mitzvah of kibud Av, it does NOT specify how many inches long a skirt must be. As long as the mother is not overes halacha, the daughter who disobeys her IS overes.
With all the nice talk about hiow beautiful this is, the gift of tznius, the inspiration, how proud we all are of the daughter who wants to be mroe tzniusdig, etc. etc. (and you are all correct), nevertheless, in your zeal to promote your particular vision of what is tzniusdig, you lose sight of the halachos that are unquestionably mandatory. For a daughter to basically blow her mother’s wishes off WHEN THE MOTHER IS NOT ASKING HER TO DO ANYTHING WRONG, is far worse than wearing the skirt two inches shorter than the daughter would like, when he shorter skirt is still well within tzniusdig range.
Daughter, please have a long and calm talk with your mom to explain how you feel. There may be a compromise here. Your mom may be feeling that you are heading in a religious direction that makes her nervous, especially if she is a frum lady. When kids become baalei teshuvah, the irreligious parent feels rejected. When an already FRUM parent sees her own frumkeit being perceived as not frum enough for her child, it is very hurtful and an even worse rejection. That happens a lot when kids come home froma year or two of Seminary. See what’s going on and try to explain your feelings to your mom, and show respect for HERS. Explore the possibility that maybe she is not wrong, and try to help her see your persepctive in a reflective and respectful way.