I'm not sure I'd ever describe Miss Maudie as being "mean" in To Kill A Mockingbird. However, she does stand up for what she believes is right, and her assertive words might, to some, come across as being short or biting.
One such incident occurs in Chapter 24, when Miss Maudie is invited to participate in Aunt Alexandra's missionary circle. For most of the meeting, Miss Maudie sits quietly and listens to the women gossip about people in Maycomb. In short, Mrs. Merriweather begins to complain that Sophy, her housekeeper, has been "sulky" lately; this is an obvious reference to her upset about the Tom Robinson trial. Shortly after, Mrs. Merriweather indirectly criticizes Atticus, in whose house she is presently sitting, by saying,
Gertrude, I tell you there are some good but misguided people in this town. Good, but misguided. Folks in this town who think they're doing right, I mean. Now far be it from me to say who, but some of 'em in this town thought they were doing the right thing a while back, but all they did was stir 'em up.
With this, Miss Maudie, who certainly does not share the missionary circle ladies' views, feels she has to speak up:
His food doesn't stick going down, does it?
Obviously, Maudie is pointing out the fact that Mrs. Merriweather has the audacity to sit in Atticus's house, eat his food, and criticize him at the same time. When Mrs. Merriweather claims she doesn't know what Miss Maudie means, Miss Maudie says, "I'm sure you do." Then, Scout explains:
She said no more. When Miss Maudie was angry her brevity was icy. Something had made her deeply angry, and her gray eyes were as cold as her voice.
Again, I'm not sure that I'd label Miss Maudie as "mean" at any point in the novel. As I said before, though, she does assert herself and stand up for what she believes is right.