If you are after dialogue that Mattie says herself that gives a description of her character, you might like to think about how she describes herself to Ethan in Chapter Two of this great novel, when she talks about leaving and not being very good at her job. Consider the following quote:
"I konw I ain't anything like as smart as I ought to be," she went on, while he vainly struggled for expression. "There's lots of things a hired girl could do that come awkward to me still--and I haven't got much strength in my arms. But if she'd only tell me I'd try. You know she hardly ever says anything, and sometimes I can see she ain't suited, and yet I don't know why."
Here we can see that Mattie describes herself as not being very clever, and also rather weak, especially in her arms. She also says that she is not necessarily good at her job, and that she is still learning to do many things that others in her place would have learned how to do much quicker. This is an important quote as it represents Mattie's own ideas about herself and how she is viewed by others.