Eh, there was a bit of teenaged huffiness built into it, too ("Why don't you tell my father? You're good at that.")... she rejected the idea of going to Ursula at first until she realized Sebastian was spying on her (oh hey, shades of Terra!). She just doesn't really seem like a pure light character in any way that wouldn't render the entire concept pointless.
Now it just seems like one it nitpicking her character, which is fine. However, you also have to remember that:
1) Jasmine snuck out of the palace often leading to her encounter with Aladdin
2) Belle definitely had a temper when she talked back to the Beast after he saved her, and she was stubborn at first about dinner, despite the fact that she was hungry.
3) Alice starts to insult the Queen after she felt that she had more power as she grew in size.
4) As seen by the trivia, Kairi is illiterate and stubborn as she won't proof-read her letters jkjkjk [/sarcasm]
So Ariel had her faults, but so did some of the other princesses as well, it just goes to show that they are human