Ok, I got to clarify that I don't actually know why the Twitter handle is like that, it was just may best guess. But maybe it indeed was intended to be a romanization of their name, and I was completely wrong. In hindsight, I shouldn't have speculated about it.

However, this doesn't change the fact that in Japanese their name is just Meika. It even says Meika on their official Upd8 page. Also, I don't watch this VTuber regularly, but I did saw some of their streams and I have never heard of them referring to themselves as Makea, it was just Meika regardless of the avatar.