This is the first time I see a Japanese publisher backtrack on skin colour, that's interesting.
FYI, skin colour issues are prevalent in Asia since forever ago.
Darker skin is still often seen as inferior to fairer skin (why do you think skin bleachers are so common here lol), and it is deeply ingrained that fairer skin = more 'expensive' or more 'noble' (or more fit to be placed in a higher caste or whatever).
In Japan, there are still this deeply rooted xenophobia towards other Asians as well even though not as much as let's say a few decades ago. This can be seen especially towards south east asians that have naturally predisposed darker skin. Heck, the weird treatment even affect sometimes towards darker-skinned Japanese people themselves.
(Gyaru is a fashion/self expression/lifestyle statement and a niche, so it is not a mainstream thing and can't be used as an 'acceptance' excuse)
So, I figure the Japanese fans who complained may have come from this perspective regardless the original source, because truthfully 'becoming fair and thus more desirable/noble' has been a sus trope since forever ago, not to mention outdated and a very deeply rooted old mindset (skin bleaching anyone?) in which the original author may have had. Not maliciously written, since author did try to instill the fact her becoming fairer showed people's hypocrisy, but the comments they wrote on her friends telling her looking more 'beautiful' after being fairer is definitely based on that deeply rooted old mindset.
The manga version story wouldn't be that damaged since it is truncated from the original anyway (for adaptation purposes), and while the message may have changed a bit, the adjustments doesn't 'ruin' the story overall ending like some of you crying out loud here.
The hypocrisy is still there, aka people being nicer to her because she is now a hero who received the light king blessings which raised her status, while before they had been superficially avoiding her because of her skin colour & its implications despite the fact she has always been the same kind person before and after she received the dark king blessings.
The people are now giving her hollow kindness due to her accolades, which is on the same vein if they became less afraid of her just because she becomes fairer.
The story isn't damaged, if anything it gives a bit more depth or made it a bit layered rather than the simple fairy tale feel.
Then, what's not to like?