I thought "Real Me," and the revelations that followed, were awesome in the way that they played with the "suddenly introducing a younger character" trope. But I agree; Dawn was mainly annoying because she was supposed to be 14 but acted and was treated like a much younger child, and the audience had a baseline for how teenagers acted within the context of the show's universe. (I work with teenagers, and I can attest to the fact that Dawn is not an inaccurate depiction of a 14-year-old, but there's real life and there's what's actually entertaining to watch.)