Is there an example of polysyndeton in "To Kill a Mockingbird"?
Yes there are many!
Here it is:
"Neighbours bring food with death and flowers with sickness and little things in between. Boo was our neighbour. He gave us two soap dolls, a broken watch and chain, a pair of good-luck pennies and our lives. But neighbours give in return. We never put back into the tree what we took out of it; we had given him nothing in return."
Just a reminder, a polysyndeton is the use of several conjunctions in close succession, especially where some could otherwise be omitted.
The quote repeats death, flowers, and sickness to emphasize a point. However, it is unneeded and the sentence could do just fine without it.