I think the way that the author, Katheryn Stockett, makes Aibeleen talk in the story is not a stereotype but just simply displaying how black folk commonly talked back then. Even in my own experiences I have come across my fair share of black people that still speak with this accent and most of them were in the east coast, which leads me to believe they were probably in southern cities/states at one point in their lifetime.  I think the idea of the author using this southern dialect is a good idea because it automatically injects the reader into the world she's trying to create thus making it easier for people to paint vivid pictures in their minds as they go along the story. At first I didn't really like the dialect because my mind would auto correct sentences and grammar because of the english text I'm used to reading, but once I gave it a couple of pages, the southern english just settled in more comfortably making reading the book a lot less annoying.