>>93496 (OP)It's not an "Osakan accent", she's just speaking in Kansai-Japanese. The Kansai area covers much more than Osaka.
And yes, it's mean to call Ayumu "Osaka" because she despises that nickname and doesn't even consider herself as being Osaka as evident by how she looks for Ayumu in that list of names for the new classes after they got shuffled. Remember "Osaka" is an invention of Tomo, the bully obsessed with making others feel bad in order to make herself feel better. Tomo wanted her to be a stereotypical Osakan, and when she realized she wasn't like one she forced that name on her. It's just Tomo stupidity.
But aside from that, calling her "Osaka" is a joke of irony for the audience based on her behaving the opposite of an Osakan. That's the actual purpose of calling her "Osaka", because it's so inappropriate. Like adding "Little" before the name of some mega fat or huge guy, for example.
And remember Ayumu isn't from Osaka. She only lived there momentarily. She lived in Kobe earlier, and she was born in Wakayama. All those places are in the Kansai region, by the way.