The only 'appropriate' diagnosis code is one that correctly reports the diagnosis that the provider has documented for the patient. But when a payer uses this denial reason, they are usually not saying that the claim was coded inappropriately, but rather that the diagnosis is not appropriate for meeting their coverage requirements for payment of the particular procedure.
L91.0 is the code for a hypertrophic or keloid scar, and injection of Kenalog is a commonly used treatment for this condition. If that is in fact the diagnosis the provider had made, then it is an appropriate code. However, many payers consider this a cosmetic procedure and will not cover it, and I expect that is likely the case here. If the documentation shows that the scar was the sequela of a covered and medically necessary procedure, such as a surgery to remove a malignancy or repair of a traumatic wound, then you might consider adding an additional code to indicate this and that could possibly meet the payer's requirements.