Oh no! Don't try to blame the readers on it.
The story was sold as comfy, without quotes.
No, it was not clear that everything was her point of view, after all, ALL stories are told in the main character point of view.
Yeah, this could be a smart twist if the author didn't decide to take 20 chapters to pull this, sorry, you can defend this all you want, but nobody can complain about reads getting frustrated for waiting a specific kind of story just to have a totally different thing.
You know, people have their reasons to pick specific genres of stories. Sometimes a reader of a wholesome kind of story don't want to read heavy stuff, sometimes those readers have really good motives for this choice (like dealing with heavy depression and wanting to avoid heavy plot).
I can say that I was this kind of reader before, now I am stable, but I can totally understand why this is not the best way to write this kind of plot twist.
I am a writer that write short horror stories, in the sense that the stories that write ends badly a lot of times, but, I would never write chapters and chapters of a cute story just to totally change the genre to eldritch horror