This is a good question. The above answer gives some good points, but there is more. If we look at the context, we learn a few things about Mrs. Dubose. From the outside she is a nasty and mean woman. And yes she does yell at the kids a lot, and she does call Scout an "ugly girl" - not to mention she is also racist. But there is another side to her that no one saw, except Atticus and a few other kind adults.
Mrs. Dubose is a sickly old woman who is addicted to morphine. She is also on the verge of dying and shortly after Jem reads to her as a punishment for destroying her flowers, she does pass away.
However, before she died she successfully battled her addiction to morphine. From this perspective, Atticus called her one of the bravest women he has ever met. Not only did Mrs. Dubose face death with courage, she also battled her addiction. Finally, after she died she gave Jem a present, a box with with a single white camellia. This showed her heart of kindness. With Mrs. Dubose, there was more than met the eyes.
Chapter 11 centers around the children's experiences with Mrs. Dubose. Jem and Scout are taught to be respectful to their elders, so Scout sweetly says, "Hey, Mrs. Dubose," in an effort to be courteous. What she gets back is a rather rude comment: "Don't you say hey to me, you ugly girl! You say good afternoon, Mrs. Dubose!" (99). Part of the reason she calls Scout an "ugly girl" is because she disapproves of Scout's overalls; and, another part might be because she disapproves of her manners/speech. Mrs. Dubose is a lot like Aunt Alexandra who believes that a little girl should not be running around looking like a boy or speaking like one. Mrs. Dubose has no qualms telling Scout, "You should be in a dress and camisole, young lady!" (101). It's possible that Scout probably doesn't look as clean as she is while wearing overalls, either. Her whole presentation is "ugly" to Mrs. Dubose because she doesn't approve of it.
The quote you are looking for is in the very beginning of chapter 11; I don't know what version of the book you have, so I can't give a page number. But look at the end of the 4th paragraph of chapter 11. We are introduced for the first time to the ironically brave "lady" Mrs. Dubose in that chapter, and the first direct quote that we get from her is, "Don't you say hey to me, you ugly girl!". So right off the bat we get a fairly surly impression of her, one that doesn't really change until we get the background information from Atticus, that she was working very hard to break her addiction to morphine. Atticus said that despite her rough exterior, she was a "great lady" with "real courage" and "the bravest person" that he knew. And she was, indeed, although a brave lady with a fierce tongue.