Sokka missed Aang.
He just didn't know Zuko. Even though Zuko had done a lot to come to them, to redeem himself, Sokka didn't know what to say to him.
For one thing, with Aang, he was never at a loss for words. For another, Aang's easy friendliness was an excellent icebreaker, rarely engendering awkward silences. With him around, Sokka and Zuko would have had something to talk about, and someone to talk with.
As it was, though, this would be a really awkward trip if Sokka didn't say something. He cast his eyes around for conversational topics.
"Pretty clouds."
This is on the airship, on the way to the Boiling Rock. Poor awkward boys.