Within the context of this manga
Black skin= ugly, evil, cruel, conniving
Light skin= beautiful, pure, kind, innocent
I personally do not like the skin change, as MC has been fighting against stereotypes assigned to her, has changed people's mindset, and taken care of problems all whilst having "dark" skin, and I would have rather more people come to see her holistically (not the "despite her skin" defense, as that is still saying that having dark skin is bad).
However, I do understand the significance of changing her skin, as this is a honest reflection of our society as a whole (the lighter the skin, the more intelligent, sophisticated, educated, wealthy, etc one is deemed to be over someone with darker skin). I honestly find it funny, if not ironic, how people are defending the author's choices. It's almost like they don't see that the author is calling them out...
The author is ultimately saying, if you are defending this skin change, you are a part of the problem. You should be highly upset, that solely based upon the color of physical features of which she had no choice, she was being treated like she was less than human. And now that her skin is lighter she is a beloved (slightly misunderstood) daughter, a valued member of society, a human being.
If you don't find this subject topical, reflective, and problematic then I can surmise where your headspace/political views lay.