I think that J.K. Rowling has a very specific writing style that's more often found in children's literature than adult literature, and I never see anyone talking about this when they discuss the quality of her writing.

The narrator of the Harry Potter books has a very very slight quirky/sly tone. Not enough to be an actual character, like Lemony Snicket in The Series of Unfortunate Events, but just subtly written in a bit of a wry or arch way. I think a lot of people really enjoy this style, because it can make a story feel very personable and like the narrator is talking to you just a little bit, and it can also add to the humor and quirkiness of the world, story, and characters.

As an example, think of the opening line of the first book: "Mr. and Mrs. Dursley of number four, Privet Drive, were proud to say that they were perfectly normal, thank you very much." That "thank you very much" is funny, and it's clever to have the narrator say it instead of having the Dursleys say it themselves.

As I said at the top, I think this style is much more common in children's literature (especially 20th century children's literature) than adult literature, but the adult author that I would compare her most strongly to is Fredrik Backman. He also uses this "sly narrator" style and I think he's developed quite a following of readers who adore his books largely because of this style and the emotional pull that his stories have. Even if you're not a fan of either author or there are elements of their writing style you don't like (and I don't always love Backman, to be honest), that style is deliberate and well executed in both cases.