The big moment that shows Scout is growing up is that she comes to the conclusion that Mayella must be lonelier than Boo Radley. White people won't have anything to do with her because of her family and poverty and black people wouldn't have anything to do with her because she is white. Scout and Dill have already come to the conclusion that Boo Radley perhaps wants to stay in his house--but Mayella seems to want to be a part of a society that won't have anything to do with her.
Scout is able to put herself in Mayella's shoes and understand what it must feel like to be Mayella. Scout is also not simply judging Mayella on what she has heard about Mayella or the Ewells. Scout is able to see that a person doesn't have to be all one thing--like Boo isn't just creepy and Mayella isn't just a rude Ewell--there are different parts to everyone's personality.
At the beginning of Chapter 18, Scout gives her initial impressions of Mayella Ewell as Mayella takes the witness stand. Scout initially mentions that Mayella seemed "fragile-looking," but when she faces the audience, Scout perceives her as a strong girl who is accustomed to strenuous labor. Scout then mentions that it is easy to tell when a person has taken a bath lately by looking closely at their skin. Unlike Mayella's father, who has scalded skin because he rarely bathes, Scout can tell that Mayella bathes regularly and attempts to stay clean. This observation is significant because it characterizes Mayella as a decent person in a rough situation. Scout's initial impressions sympathize with Mayella, who has a lived a rough life growing up with an abusive alcoholic. Scout's ability to sympathize with and show empathy for Mayella illustrate her growth as a character. Scout has learned to view situations from different perspectives and applies her earlier life lessons when she examines Mayella at the witness stand. Scout's ability to notice small details and draw conclusions also illustrates her maturation and growth.