The real answer to your question is not simply that Scout wears dresses or not. The real answer is that Scout grows up and is able to do what Atticus has always wanted her to - step inside someone else's shoes and see what life looks like from their point of view. Scout achieves this when she walks Boo home the night of the attack. After dropping him off, she sees the window that he has looked out of for years. This is same window that he has peered out and looked at the kids and watched them grow. By doing this, Scout matures. She is no longer the same impetuous girl who would hasten to beat up Cecil Jacobs. So, in that way, she has grown up.
As for whether or not she wears dresses, well the ladies' dinner party is a good example. Yes, Scout wears a dress, but she has her jeans on underneath. This is a great way for Harper Lee to show the reader that, yes, Scout is ready to grow up, but, no, she is not ready to do so just yet.
This is echoed at the end of the book when Scout tries to stay awake with Atticus as he reads and looks after Jem. She is trying to be an adult here. But she falls asleep, and, like a little child, Atticus carries her to bed.
So she has grown up some, but she is not ready to become a little Aunt Alexandra.