It is fully permissible to dry oneself off with a towel as usual on Shabbat. The main caveat is to not to wring out the towel or hair. Patting the hair dry and even rubbing it dry, as necessary, are permissible.
See
here for our article on Mikveh on Shabbat and Yom Tov.
Please let us know if we can be of further assistance.
Shabbat Shalom!