This is darker than I expected.
(No pun intended. Though, I suppose she also is in fact darker than I expected, technically, given that she clearly does not have any sort of dark skin on the cover XD)
But it is not falling into throes of melodrama. The plot isn't tripping over itself trying to make things more horrible. The protagonist more or less downplays everything as being normal (if regrettably so) to her experience, as one often does when things have been bad for a long time. All in all, it seems a more natural and convincing of a portrait of a human being in difficult circumstances.
Actually, there's a lot to be said for what the author does with matters of perception and character perspective here. You can see in interactions between her and those that are terrified of her, both the calm way in which she might perceive a conversation, and yet at the same time how the other party construes the same interaction as being full of suspense and menace—because they treat her quietness as one would treat teatime with a demon lord: Groping for deadly, unnamed subtleties—but there actually are none.
(Her high-society bullies, by contrast, are unconvincingly shallow paper-cut-outs. They make no sense. Not everything in this is masterfully done, I guess XD)
TL;DR: My standards for good writing in isekai are naturally low, but, this has some rather well-done bits to it.