On the contrary, those characters are quite plausible as the book is somewhat autobiographical. Maybe Atticus is a bit romanticized, moreso in the film than in the book.
Scout is based on Harper Lee (herself) and the quirky neighborhood friend is based on Truman Capote. They did grow up together.
That being said, I LOVE the story, and consider Atticus the epitome of the Southern Gentleman, tough with those who need it, compassionate to those who deserve it, and always mindful of justice.
As for the characters, the black people in the movie, when met with kindness respond accordingly. Otherwise, they kept clear of the folks who they knew would treat them badly. The housekeeper is like so many black women of that era. They had their own families, but because of the time spent with the children of their employers, helped raise theirs as well. In many cases, they were loved as part of those families.
My hubby, SirKit grew up in the MS Delta, and his Mama always said that the South adjusted to the integration of whites and blacks much more easily because we had actually known and loved individual black people as we grew up. The transition was not as jolting as it was in other areas of the country, where there were some whites who had never ever met a black person before adulthood.