It is always important that critiques of narratives and characters appropriately consider both the era in which the novel has been written and the setting of this novel.
- Overalls as symbol
So, since Harper Lee's To Kill a Mockingbird is set in the South of the 1930's, a time in which women occupied only the domestic or social worlds, and there were certainly clearly defined gender roles, Scout's wearing of overalls--and even being called "Scout,"for that matter, indicates true rebellion of social mores, one that appalls Aunt Alexandra when she comes to live with her brother and his family.
And, yet, overalls befit Scout, a little "tomboy" as such girls were called pre-Gloria Steinem. She plays with boys, her brother Jem and Dill, and she can fight quite well. The overalls symbolize Scout's sense of equality with any boy aa well as her independence and spirited nature. She does not play with dolls; instead, she likes to pull pranks and run, wrestle, and shoot a BBgun and play outdoors as the boys around her do. The fact that she has an older brother influences Scout's attitude about fun, of course.
It is rather curious that it is Calpurnia who first insists that Scout wear a dress, at least within the narrative as, oddly, there is no mention of Atticus and the children dressing up for attendance at church on Sundays. Calpurnia insists that Scout wear a dress with a starched pinafore because she does not want anyone to think that she does not take good care of the Finch children, so Scout's usual habiliments would signify neglect. But, Scout's wearing overalls at home to play may simply signify the practicality of Atticus who cares less for convention and more for his daughter's knees and comfort. Perhaps, too, allowing Scout to "dress down" lets people know that the Finches do not "put on airs" and feel themselves superior to others who suffer under the Depression. Then, after Aunt Alexandra arrives, she demands that Scout dress up for her Missionary Tea as she wants "to keep up appearances." Scout sits in her "pink Sunday dress, shoes, and a petticoat."
- The Mockingbird
When the children receive their air-rifles for Christmas, Atticus instructs them,
"Shoot all the bluejays you want, if you can hit'em, but remember it's a sin to kill a mockingbird."
Of course, this introduces the symbol that extends to anyone or anything that is innocent, such as Tom Robinson. After the trial Mr. Underwood writes an editorial in which he uses the symbol of the mockingbird to represent the innocent victim of injustice.
Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of songbirds by hunters and children....
Finally, after Bob Ewell is killed and Sheriff Tate and Atticus confer with one another about the fact that Boo Radley has defended Jem and Scout, Sheriff Tate suggests that they simply say that Ewell fell on the knife that killed him rather than expose the recluse to questioning and a hearing. Atticus looks at Scout and asks her what she thinks,
"Well, it'd be sort of like shootin' a mockingbird, wouldn't it?"
Atticus understands and recognizes the wisdom of his daughter's remark as Boo is a true innocent.