I think one of JKR's strongest suits, in both HP and the Strike series, is how she captures real people. All of her characters feel like people I could meet on the street or share a cup of coffee with. I can imagine what our conversation would be like. There's a lot I find admirable about her writing, but this in particular is something I appreciate a lot more now that I'm reading her as an adult. She seems to genuinely care about real people, which translates to her writing.
Yes, i admire that quality of her as well, especially because her characters feel real despite being… not exactly stereotypical but… easy to read, easy to sumrise. You can't call Vernon Dursley stereotypical because the details of him are so specific, but the details are all that's needed.
At the same time her physical descriptions are minimalist, only including what's useful for either plot or characterisation or (this one is more present in Strike) social background and education. If she were a painter she'd be a great portraitist, a fast working one.
And yes, to segue back to that one interview of Le Guin… sometimes those "portraits" of unpleasant imaginary people can seem a bit mean, because they feel so real.
One example is "Hartella" in The Ink Black Heart. Hartella is not the most hateful character in that book, but she is probably one character many tumblr users could see themselves in, as a "superfan" of a show, whose real name is known in the fandom due to being minorly involved with the production. She's an opportunist, neither smart or talented, she's needy and insecure, she's a very bad judge of character, a coward, lets herself be manipulated into hurting other people, easily joins in bullying to fit in, etc. She's also described as white, fat and unattractive, and having an annoying habit of ending all her sentences in a questioning tone even if they're not questions (subtext: because she's so insecure and unwilling to develop her own critical thinking, she can never make statements).
I remember a few readers took offense at her being described as fat and unattractive specifically. And yet… does this description not feel like a real person? I know i have met that specific girl online at least once for sure (in fact the resemblance is uncanny, down to the speech mannerism). I've known girls who partially looked or sounded like Hartella. I guess that can sting.