It's a somewhat common trope in TV and Cinema (Independance Day comes to mind, as well as any horror movie taking place near the south, The Hills Have Eyes and

It's a somewhat common trope in TV and Cinema (Independance Day comes to mind, as well as any horror movie taking place near the south, The Hills Have Eyes and Texas Chainsaw Massacre for example) to give characters that are shown as stupid/misadapted a Southern accent.