Scout’s use of overalls is symbolic because they demonstrate her independence.
Scout is a girl, but she hangs out with boys more than girls because there are not many kids to play with in her neighborhood. As a result, she is a tomboy. She likes to wear overalls because she can’t “do nothing” in a dress.
Aunt Alexandra tells Scout that she should be a perfect lady and a “ray of sunshine” in her father’s life.
I suggested that one could be a ray of sunshine in pants just as well, but Aunty said that one had to behave like a sunbeam, that I was born good but had grown progressively worse every year. (ch 9)
This hurts Scout’s feelings and upsets her. She likes to run and play. She even fights like a boy. It is a part of her personality. She does not appreciate Aunt Alexandra trying to turn her into someone that she is not.
Scout marches to her own drummer. She has a unique way of looking at the world, and that is reflected in her manner of dress. She is not a bad kid, but she is an opinionated one.
On another level, the pants symbolize rigid gender roles in the South at this time. Scout is expected to look and act a certain way. If she does not, she is not a lady and that is terrible. Scout does not like being pushed into a box any more than Tom Robinson or Boo Radley.