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.
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.