She listened to what Zuko had to say and tried, really tried, not to react in any particular way. Reminding herself once more that he wasn't Sokka, or Aang, or Toph, Katara finally questioned, "Why are you wearing that mask again?"
She wanted to point out that, if Iroh didn't approve, Zuko should know he was doing wrong. She wanted to say that he had no reason to hide any part of who he was, especially not in this place where so many people cared for and accepted him no matter what he might do. She wanted to say a lot of things... but she stuck with that one question. Because until Katara knew the answer to 'why', she couldn't begin to say much of anything else.