Even if she had been able, I don’t think Katara healing his scar would have made much difference. Although it was a physical manifestation of his pain and failure, Zuko’s scar also carried deep emotional and psychological barriers that Katara would not have been able to wash away. So I still think that even if Katara had healed his scar, Zuko probably still would have sided with Azula because he had not yet understood that his destiny didn’t lie with supporting his father.