From what I understand, she had a very hard time meshing into Japanese society when she and her family moved back over there when she was....13, I believe? From then all the way until maybe about right after she started streaming for Cover. Her Japanese was absolutely god-awful (you can hear how awkward and horrible it is in her pre-Hololive Kson streams/convention appearances, and even her very early-on streams as Coco), and she couldn't find a 'real' job because no one would hire her due to her goofnuts Southern-American-mixed-with-gook accent and absolutely sub-par Japanese speaking skill.
I'm not 100% sure this is true, but I've read from multiple sources who would probably know that Kson originally applied to be on the translation team for Hololive, but was convinced by god-only-knows to audition for a talent slot proper.
But anyway, I feel like it's no secret that even Japanese-people-who-weren't-actually-born-in-Japan are treated with a twinge of disrespect over there, it definitely can't be easy. I feel like they may have it worse than people of a non-Japanese persuasion, especially if the natives don't realize right away that you're not "really" one of them, doubly so since I know foreigners have at least a small handful of laws (and other "unspoken rules") that make their lives harder just living there.
I dunno if any of the whole "Coco is trying to bury her American side" theory holds water though. I mean, she's definitely starting to skew a bit more towards the JP audience, but I think that's just to compensate for how EN-centric she was in her earlier days. But there's no way she's gonna drop it considering how ridiculously massive her overseas viewerships are. I'm not sure what would make someone think her English skill is "deteriorating," I don't see nor hear that at all.