Why does Miss Maudie compare watching the trial to a Roman carnival?
A Roman Carnival has been defined as an event which resembles Mardi Gras. A tradition that dates back to the Middle Ages, this celebration is, to this day, a popular event in Italy, and other countries such as Brazil (whose carnival in Rio is the largest in the world).
Traditionally, Roman carnival consisted of a large public celebration that lasted 8 days, ending the night of Fat Tuesday, the day marking the beginning of Lent . . . During Carnival, the most famous streets and squares would be filled with spectators captivated by the shows, music, masked processions and stands selling goods.[http://www.welcometorome.net/en/about-rome/roman%20culture/roman-carnival]
Miss Maudie's allusion is an apt one, as people from all over the county come to Maycomb's square where the courthouse is located. The parade of odd folks in wagons and other residents of the city set up "picnic parties" on the lawn. When Dill asks Miss Maudie if she is not going, this sensible woman answers that she refuses to engage in such a morbid activity as "watching a poor devil on trial for his life." (Ch.16)
Interestingly, too, is the fact that during the Roman carnival, pigs were herded to an area where there was a cliff. There these animals were forced to jump to their deaths, and the Romans were allowed to collect them and take them home to eat. This practice suggests the concept of a sacrificial animal, which is not unlike what Tom Robinson later becomes for the Jim Crow Southern town.
Miss Maudie says this in Chapter 16.
I think that she is comparing the trial to the gladiatorial contests that Romans used to watch. I think that she is saying that the people of Maycomb are like the Romans were. I think that what she is thinking is that both the Romans and the people of Maycomb are getting entertainment by watching other people fighting for their lives. She thinks that it is disgusting to do that sort of thing for fun.
I'm not convinced she's right, though. The Romans watched other people that they didn't even know dying. The people in Maycomb actually care what happens -- most of the whites probably think it will be a good thing if Tom Robinson is killed. They think it will preserve their way of life. So I don't think they're just watching for fun the way the Romans did.
Watching the small town of Maycomb gather with crowds of spectators arriving to see the trial of Tom Robinson reminded Miss Maudie of a carnival atmosphere. A wagonload of Mennonites rode by, followed by another wagon full of "stern-faced citizens" who heckled Maudie with a bible scripture; Maudie defiantly responded with one in return. Dolphus Raymond rode his thoroughbred; X Billings passed by on his mule. The public hitching rail soon was full. "The courthouse sqare was covered with picnic parties..." Children played games and babies suckled at their mothers' breasts. Other friends and neighbors gathered on the street to gossip and speculate about the trial. For such a solemn and serious event, it seemed like a carnival to Maudie.