Is Mr. Cunningham a protagonist or an antagonist in Harperr Lee's To Kill a Mockingbird?
A protagonist is defined as a central character in a story who grows and changes as a result of resolving the conflict. Since a protagonist can only be a central character, we know Mr. Cunningham in Harper Lee's To Kill a Mockingbird can never be considered a protagonist.
In contrast, an antagonist is any character or group of characters that opposes the protagonist and fights to prevent him or her from fulfilling goals. Based on that definition, we could make a case that Mr. Cunningham is an antagonist.
We most clearly see Mr. Cunningham as an antagonist when he faces Atticus as leader of the lynch mob, attempting to fulfill what the mob sees as justice by taking Tom Robinson's life before he can stand trial. We know Mr. Cunningham and his clan oppose Atticus because Atticus sees Robinson's innocence and is bent on doing his utmost to defend him in trial, whereas Mr. Cunningham and his family see Robinson as guilty due to the color of his skin. We see Mr. Cunningham oppose Atticus when he arrives at the jail with his clan and says to Atticus, "He in there, Mr. Finch?" Another member of the group says, "You know what we want.... Get aside from the door, Mr. Finch" (Ch. 16).
Though the scene turns out all right because Scout brings Mr. Cunningham to his senses by reminding him of his humanity and of how much he values Atticus, Mr. Cunningham and his family members oppose the protagonists in this scene, and so are (at least temporarily) antagonists.
It's important to note, however, that in general To Kill a Mockingbird emphasizes how difficult it is to place labels on anybody—including the designation "antagonist." There are few irredeemable characters in the story. The novel forces readers to confront the idea that even Mr. Cunningham and his family, who are prepared under these circumstances to commit murder in cold blood, are human beings capable of honor and compassion. Despite his poverty, Mr. Cunningham works hard to repay his debts; he brings Atticus produce from his farm in order to repay him for his legal help, and his son Walter is careful never to accept money from anyone if he thinks he might not be able to pay it back (as evidenced by his refusal of Miss Caroline's offer of lunch money). Similarly, a Cunningham is the only juror to consider acquittal after hearing Tom Robinson's testimony—he's the one who keeps the jury out so long.
In short, though we can argue that Mr. Cunningham is sometimes an antagonist, it's important to recognize that he, like many of the characters in To Kill a Mockingbird, has both good and bad qualities; sometimes he fights against the protagonists, but sometimes he fights for them.