There is nothing fake or wrong about the way Carrie speaks. It sounds American all the way. Posh American when she is being a diplomat and normal when she isn’t.
No the problem i have with it is it does sound right it’s kinda jarring. Also how do you explain the first scene with her talking to vader that’s kinda jarring and when han reunites with leia and luke near the end of the movie she uses it so it is jarring. Think about there was nothing diplomatic in that scene unless i’m wrong. Also the vader scene come on that is very very jarring i don’t know.