Yeah, for all we know, a lot of the Air Nomad could have been just like Aang. Look at Gyatso, he threw pies at the monks, and he did it for the lulz.
Once and while, Aang's all "I gotta do this alone!" (301 for example) but Katara didn't accept that and quickly forced that idea out of him. Aang has gotten better goofing off, and the most recent time he was messing around was "WAT" and that was because their plan was ruined before, and Aang felt a little discouraged.
He was every right to be a little childish, if he was wasn't, the stress would probably drive in insane. Just look at "NaD". But he also has gotten a lot better about knowing when to be serious, and when to have fun.
And that is just my .02 about why Aang is a good character.