The usual spelling of mockingbird author Toni Cade Bambara uses in her title "Blues Ain't No Mockin Bird," as seen in her volume of short stories the work was first published in, tilted Gorilla, My Love, shows us that she is using the word mockingbird in a symbolically unusual way. Mockingbirds are highly intelligent, trainable birds; they are also highly protective of their young. Therefore, mockingbirds typically symbolize intelligence and courage, especially in Native American folklore ("Native American Mockingbird Mytholoy"). They also bring pleasure through their music, so they are known to symbolize joy and communication. However, Bambara is using mockingbird in her title to point to the wrongdoings of the men in her story trying to photograph the Cains in their state of poverty. In doing so, she emphasizes her central themes concerning lack of respect for others and lack of compassion. The key is noting that her chosen unusual spelling of "mockin bird" emphasizes the word mocking rather than just pointing to the actual bird.
To mock is to make fun of someone, to laugh at someone, and to ridicule the person for doing stupid things. We can mock people by "saying unkind things about them" (Longman Dictionary of Contemporary English Online). We further mock people by showing them contempt, which is the feeling of meanness that stems from thinking people are unworthy or worthless, a feeling that goes hand in hand with racial prejudices (Random House Dictionary). We can also mock others by imitating them (Random House Dictionary), just as the mockingbird imitates the cries of other birds, even of cats and dogs.
A film can be seen as a facsimile of reality. The men in the story with the camera trying to film the Cains in their state of poverty are trying to create a copy of the Cains' life thereby trying to capture an imitation of them, an imitation they intend to show to the county officials. In trying to imitate the Cains, they are behaving like mockingbirds. More importantly, they are mocking the Cains through their feelings of contempt for the Cains.
We see the men's sentiment that the Cains, as well as others in the same socioeconomic class as the Cains, are worthless and unworthy individuals when we see the men say they are there filming for the county as part of the food stamp campaign. One of the men makes the following very important statement:
I see you grow your own vegetables ... . If more folks did that, see, there'd be no need--
Though he doesn't finish his sentence because Granny silences him with her facial expression, the reader can easily see he was about to say that if more people in Granny's socioeconomic class grew their own vegetables, there would be "no need" for the county to waste its money by spending it on food stamps for the poor. From his statement, the reader can infer the two men think people of Granny's socioeconomic class are unworthy of care and assistance, a sentiment that fits the definition of mocking someone.
Hence, "mockin bird," as Bambara spells it in her title, represents cruel mockery, and she is using her title to say that the blues, or people's states of depression from poverty or otherwise, cannot be truly captured by imitation and are nothing to mock.