Using the common motive of "making the show appeal to a wider [American] audience," it seemed to me that they wanted her character to hit closer to home than a polished young lady would. Of course, my opinion is just a shallow one - a thought in passing.

Thinking on it now you are probably right to question me on my association of 'rude and spoiled' with 'American,' but that was just my gut reaction. FWIW, I'm an American with minimal knowledge of the demeanor teenage girls from other societies, Western or otherwise.