Well, in the English translation of the Azumanga Daioh manga, Osaka has a New York accent, and it's terrible. Much worse than the Southern accent.
I think the Southern accent is fine for people from Osaka, as long as the accent isn't too heavy or done by a horrible voice actor.
And it has nothing to do with how the rest of the world sees America. As Arizth said, it's about giving a character, that has an accent in the original Japanese, an accent in the English dub. Imagine if Osaka talked exactly the same as everyone else; her being different wouldn't be as obvious. Plus, it's not always explicitly stated that a person is from Osaka, but the accent would give it away in the original Japanese. There needs to be an equivalent of that in the English dub.