I can speak from direct experience that vast numbers of Americans, including many from the South, believe that the so-called "hillbilly" accent of the Appalachians and Upper South indicates markedly low intelligence and general crudity.

But they think the same thing about the working-class NYC (erroneously called "Brooklyn") accent, the Gulf accent (progenitor of the stereotypical African-American accent), as well as several others.

(The above simplifies considerably the American accent situation.)

The general "rule" is, the stronger and more identifiable your regional accent, the dumber certain half-educated people (not usually academics) will assume you are.

Does that "rule" not apply in Britain as well?