Personifying the Mockingbird:

The Mockingbird known for the habit of some species mimicking the songs of insect and amphibian sounds as well as other bird songs. Hence, people would see them based on what they hear and not what they see or know about it.
Just like the Mocking bird, Mrs Henry Lafayette Dubose was not what people thought she was suppose to be, but actually had another side of her which nobody knew. Nobody in Maycomb had actually see for themselves the real conditions of Mrs Dubose, therefore assuming that she is a cantankerous person from when they meet her when walking pass her house.
She had been known to be a grumpy old lady in the novel. The children did not like her. And everytime they passed by her house, she would throw criticisms towards them and also about Atticus defending a black in court.
However, Mrs Dubose is a morphine addict, and therefore was not herself. She herself did not really know what she was doing.