Guess I'm just yappin about Clover's accent today.
Her accent is indeed real, but she just overplays it to lean much more into the cowgirl persona.
Normal Phase: "I am the toughest and bravest cowgirl in the land."
How Clover says it: "Ahm thuh toughest eend bravest cowguhrl een thuh land!"
How Clover would say it if she wasn't overplaying her accent: "Ahm thuh toughest and bravest cowgirl in thuh land!"
Only some slight changes from over exaggerating but still changes.
Clover got ridiculed often for her accent growing up and started to hide it (but struggled to do so). During any run where she made it to the Wild East, she would stop hiding it and end up over exaggerating for the cowgirl persona. (Or well, I guess cowboy since this would be before her transition.) Post life on the surface she just starts sticking to the over exaggerated accent as the cowgirl persona is somewhat an escape tool she uses from her trauma/guilt.
Chara hated the accent at first when they met for the first time. Even when they became friends it didn't really like it and took time to warm up to it. Chara eventually finds it charming when they start developing a crush on Clover.
Chara slowly starts picking up some of Clover's accent but never to the extent Clover has it. Generally just sometimes saying "y'all" or "ahm".
Clover's thoughts do not have an accent for some unknown reason to her.