It's a Korean and MOST old society with a cast system standard of beauty, because a tan skin means labor in the sun, even more if most nobles are fair skin it also means she resembled a mixed or another race what is not well seen in a place who values bloodline (if both the lupus God and his blond hair lover had fair skin, then a fair skin must mean closeness to this bloodline as well as MC eye collor who is considered pretty and most probably blond hair)
Beauty is a product of the era, just as a big structured nose, mono eyebrows, a small feet, black teeth, and a dead appearance was/is considered the standard of beauty somewhere
Note that duke and his nephew are considered ugly just because they had freckles (which was considered ugly for a long time until western society decided skin damage was cute, and this is very recent too) the same way the maid who had pimples who in modern society is considered ugly and unseen too
What a vain story. The fact that part of her "ugly" routine is applying tan make-up? Lol.