Here's a link to the Jim Crow Museum's Page on the Mammy caricature. Here's some of what it has to say:

From slavery through the Jim Crow era, the mammy image served the political, social, and economic interests of mainstream white America. During slavery, the mammy caricature was posited as proof that blacks -- in this case, black women -- were contented, even happy, as slaves. Her wide grin, hearty laugher, and loyal servitude were offered as evidence of the supposed humanity of the institution of slavery