I'm not calling them one-dimensional, of course everypony has flaws. My example with Pinkie was a poor one, I admit. But what of the other ponies? They are the Elements of Harmony for a reason, they are the epitome of certain aspects of goodness, to have a latent opposite within them is almost unfathomable. Apple Jack is the epitome of honesty, she was forced a trait of dishonesty. Fluttershy is the epitome of kindness, you never see her wishing harm on others, she was forced a trait of hurting others.