The Romans were known for their barbaric public fights in the Roman Colosseum where gladiators fought each other and many people were sent to their death. Another form of entertainment for the Romans was to feed Christians to lions in the arena. For Miss Maudie, who is sensitive to racial issues and the behavior of people, it must have seemed like Tom was being led to his slaughter by the town’s people. People from miles around Maycomb came to see the spectacle, not only because of their curiosity and desire to be part of the action, but also because they were racist. Since the town had already deemed Tom guilty, they wanted to see Tom Robinson pay for his heinous crime of supposedly raping a white woman.
Miss Maudie is also commenting on how people revel in the pain of others. To the people attending the trial, it was entertainment. For Tom Robinson, it was life or death.
The Romans were known for their exciting but barbaric celebrations which catered to the lowest instincts in humanity. For example, armed gladiators would fight one another or wild animals, and the crowd got to see blood and slaughter! To call Maycomb that is a shorthand way of saying there's a lot of people here, they are messy and loud, and the trial is bringing out the worst of them.
They are in town for the spectacle of the trial. It's a lurid topic: a black man accused of raping a white woman. They are there for the slaughter, and to see Atticus take on the town.