I think our modern concept of "degraded" dialect is very skewed. Standardized spelling is a pretty new invention, and mostly an urban phenomenon near the time of Daniel Webster. The newest educational materials were unlikely to find themselves into rural schools very quickly. Until then, there was no such idea as spelling "wrong."

But just because a society or segment of society is not literate or is only partially literate doesn't mean they couldn't speak or think perfectly well. I think it's wrong to necessarily jump to the conclusion that the exaggerated spellings used to indicate accent is an accurate representation of their speech patterns. For example, my great-grandmother, who was never more than barely literate at best, has always had excellent diction and grammar. Way better than mine :) (Of course a mountain farm girl being illiterate was hardly an impediment at the time.)

With radio and TV our regional dialects are homogenizing -- I think it's probably safe to say that dialects were probably stronger then. But war-time songs are also propaganda, and more likely emphasize differences than try for accuracy.