What they did with Osaka in the English dub of Azumanga Daioh wasn't TERRIBLE, but it just didn't sound that appealing to me.
I think the way they portrayed Osaka in English was fantastic. The goofy southern accent fit her very very well and it gave English audiences an idea of what she's like (being from a different area then the other characters and having an accent). When you listen to an osakan accent as opposed to regular japanese while watching subs it doesn't make much of a difference to english speakers. Giving her a southern accent helped english audiences appreciate a lot of the jokes involving her.
By the way I love Osaka She is the best character in the whole Azumanga Daioh series and among the best anime characters ever.