Someone explained it pretty well a while back (albeit in a manner much more hostile than I felt was necessary): people sometimes complain when Miku is drawn as a POC because she's Japanese. How many Japanese people do you know that have naturally blue/green/teal/aqua(marine)/etc. hair that goes down to their ankles? If you can accept a Japanese person that doesn't have a naturally-occurring hair color, why not a Japanese person who doesn't have light skin?