Is it normal for a c-section scar to feel numb a year after the birth of the baby?

Can be. When the incision is made, the small nerves in the skin surface are cut. The sensation in the area may never be exactly the same.
Yes. Dr grootwassink is correct. You may find that seeing a chinese medicine practitioner and using herbs/acupressure/acupuncture can help improve the numbness and help restore the superficial nerve connections.