I suspect that Vathara's opinion of the Gaang is based on how they reacted to Zuko asking to join them in season 3. Toph was the most willing to give him a chance, so she's the voice of reason. Katara refused to even entertain the idea at first, so she's the designated unreasonable one. Sokka, who didn't like the idea of Zuko joining but was slightly less upfront about it, gets to be placed in the middle.