As a general matter, a minor only needs one parent's consent for a tatoo (or pretty much anything--e.g. travel). The divorce would not change that unless there is something in the divorce decree or settlement which could be read or interpreted as requiring your father's consent (e.g. dual consent to a "medical procedure"--while not technically a medical procedure, since it does involve a permanent body modification, an argument can be made that if the consent are both are required for something involving your health or body, that would extend to tatoos).