My mother has undifferentiated nasopharyngeal carcinoma, she finished her chemo, but it's still there. Her doctor says its ok, should I be worried?

Hard to say. The usual treatment for this disease is chemotherapy radiation and occasionally antibodies. I am not sure what was meant by "still there". I suggest that you ask her doctor to explain. I would expect that you will get the answer you need.
Depends on stage. Sorry to hear about your mother's diagnosis. Usually chemotherapy alone is not the only treatment for nasopharyngeal carcinoma, and patients will have had surgery or radiation as well. You don't tell us what stage her disease was. A chinese study showed that most patients with nasopharyngeal cancer treated with radiation and/or chemo did not have a recurrence after two years. Hope this info helps.