What is the significance of Scout being a Ham in the school play?

I wouldn’t say that Scout’s ham costume has special significance. Though To Kill A Mockingbird is a novel filled with symbols, Scout’s ham costume doesn’t have the same emotional resonance for the reader as the mockingbird, a representation of senselessly violated innocence, or Boo Radley, a symbol of Scout’s growing maturity.

I would point you in the direction of juxtaposition, not symbolism. Juxtaposition occurs when an author closely sequences two objects, events, people, or ideas...

I wouldn’t say that Scout’s ham costume has special significance. Though To Kill A Mockingbird is a novel filled with symbols, Scout’s ham costume doesn’t have the same emotional resonance for the reader as the mockingbird, a representation of senselessly violated innocence, or Boo Radley, a symbol of Scout’s growing maturity.

I would point you in the direction of juxtaposition, not symbolism. Juxtaposition occurs when an author closely sequences two objects, events, people, or ideas in order to emphasize their contrasting qualities and heighten dramatic tension.

The chapters before the school’s Halloween festival are filled with serious drama: the guilty verdict, Tom Robinson’s death, and Bob Ewell’s threats against Atticus. Jem and Scout are experiencing the most emotional moments in their lives; both feel betrayed by the world and anxious about the future. In contrast, Harper Lee draws out the comedic aspects of the Finch children’s life during the Halloween festival scenes. They are still children, after all, and sometimes concern about an embarrassing costume or the possibility of being seen together with a younger sibling can take precedence over even the most serious concerns. Consequently, this chapter is filled with slapstick humor. Scout’s ham costume is ridiculous, and when she’s forced to make a grandiose entrance by her overly enthusiastic teacher, things quickly fall apart. It’s a brief return to childhood after harrowing events. It reminds us of the scenes before Tom Robinson's trial, when Scout still had a complete child's view of the world. 

However, the return to innocence doesn’t last for long. These silly and innocent events are followed by Bob Ewell’s attack. Scout’s ham costume makes it hard for her to fight back, and she almost witnesses Jem’s death. Consequently, what was fun and silly turns deadly because of Bob Ewell’s thirst for revenge.