When you whitewash a character you are explicitly stating your vision of beauty is better, more natural, and more suitable for the audience of this fandom than whatever skintones or features were originally depicted.
When you whitewash you are quite deliberately letting those of that skintone, race, body type and facial features that we are not desirable— that we are imperfections that must be fixed.
When you whitewash you assert the idea that your standard is better. Your standard is more normal. Your standard is superior.