Well, Sora looked really childish in the clothes of Kingdom Hearts 1 and Kingdom Hearts: Chain of Memories. But this was just because he was younger, I guess... In Kingdom Hearts 2, though, he looks much better in his new outfit and with the spikier hair, and has a more mature attitude overall, which does make sense, since he's an adult now.
If Kingdom Hearts 3 revolves around him, too, I bet (and hope) that he's gonna wear even better clothes..