Miss Maudie is referring to Miss Stephanie Crawford's proclivity toward knowing everyone's business because she makes sure that she does--or at the very least, she engages in gossip and rumor mongering, and seems to think it is absolutely necessary to know everything that is going on in the community and share it with anyone who will listen. Dressed in a hat and gloves on her way to the Jitney Jungle, or so she claims, Miss Stephanie answers Miss Maudie's query and mentions a bit too casually that "I thought I might just look in at the courthouse to see what Atticus's up to." Miss Maudie's verbal irony/sarcasm is apparent when she says, "Better be careful he doesn't hand you a subpoena," referring to Miss Stephanie's role as the town busybody. Scout comments, "We asked Miss Maudie to elucidate; she said Miss Stephanie seemed to know so much about the case she might as well be called on to testify."
In To Kill A Mockingbird, Miss Maudie is different from most of the other Maycomb County residents and supports Atticus in his defense of Tom Robinson. Scout and Jem spend time with her, as long as they avoid her prized Azaleas, and Scout sometimes uses her as a sounding-board when she needs explanations for things she otherwise does not grasp because Miss Maudie will never judge her and will be honest with her. Miss Maudie recognizes the duplicity of the other residents and their hypocritical behavior. She is not a "foot-washing" Baptist and loves to spend time in her garden, even if it is frowned upon, maintaining that time indoors is just "wasted."
Harper Lee uses many literary devices in her novel which enrich the text and provide the reader with a deep understanding of matters which otherwise may be ignored. She sends a clear message of the unjust and illogical sentiments expressed by the Maycomb County residents. Miss Maudie's sarcasm allows Lee to criticize society's failure to recognize its own shortcomings: it is too busy finding fault with others.
The courthouse itself is a contradiction with Greek and Victorian elements reflecting "a people determined to preserve every physical scrap of the past" (ch 16). Miss Maudie refuses to go and watch Tom's trial because she knows that there is unlikely to be any justice for him and, when she sees Miss Stephanie's outfit, she knows she is hardly dressed to go, as Miss Stephanie proposes, to "Jitney Jungle." When Miss Stephanie tells her she is going to check how the trial is going, Miss Maudie tells Miss Stephanie to be careful that Atticus "doesn't hand you a subpoena."
This is sarcastic because Miss Stephanie thinks she is an authority on most things and despite thinking she knows everything, she knows nothing of the real problems in this trial. The sarcasm confirms the fact that Miss Stephanie's own evidence (hearsay) would be as good as any in this trial because it is not based on fact but on hearsay, supposition, prejudice and stereotypes. In other words, the so-called "evidence" is flawed and gossip serves as evidence.