I have to say I agree. I think she has an amazing imagination, none could knock that, but she isn't a great writer as in what she actually writes is often clumsy.
She improves between book 1-2 and book 3 she hits it. Really good, she's on top form. But then the editing falls apart after that. There's plot holes that don't go anywhere and the books are too big. A good writer would have been able to pare that down and produce a tighter book that was better for it. That is at least partially the publisher's fault as they should have helped edited it, and they certainly should have told her.
It's kind of the opposite of Dick King-Smith. He's a good writer and he has great ideas. But often his imagination doesn't stretch far enough. Why would you have the sheep pig accused of worrying sheep? Pigs do not worry sheep (they worry me if they're big!) so it isn't a valid accusation. Have them considering whether he's good bacon, or a dastardly plot to sell "sheep pig sausages". If nothing else I haven't ever read a book about a sheepdog that doesn't include them being accused of sheep worrying at some point, so that makes it rather boring.
If you look at the first book, it's a slow start and some of the sentences are just not quite right, and you get things that feel a bit try too hard-like Dudley's uniform, it's totally OTT.
Character wise, I think there is an aspect of using fairly standard stereotypes. The hard done by main character, the know it all (who often is a girl and they normally start off by disliking) person and side kick who sometimes is jealous of MC, but other than that is fairly bland.
And there's inconsistencies. She describes Harry's home life in a way that would be surprising if the school at any rate hadn't called social services-but the Dursleys are very much about the outward appearance, so they'd have been "oh, my poor darling orphan nephew and I'm so glad I was able to help, in public" and nasty in private.
Equally well it's unlikely Harry would have had enough confidence after that to say "there's no need to call me sir, professor" to Snape in a new school the first time he's met a teacher. He's used to bullies, he hides from them because he's learnt he's constantly the victim. He's not going to stick his neck up when he's got a chance of leaving that behind.
The other thing that surprised me when she was talking about being published was how little she had researched into doing it. She talked about not receiving the manuscript back (which was standard), that she'd sent the whole manuscript off (most places ask for max 3 chapters) and a couple of other things that I knew, having never submitted a book nor been involved in it, were things publishers said as standard.
Then if you see one of the rejection letters sent to her when she tried the adult book under a pseudonym, she clearly had still not looked at information about submitting as one of them helpfully tells her how to submit in a way that publishers want. (like the above).
I think she seems to be a great person, stands up for what she believes in. I think she is as good a writer as many out there.
But a great writer? No.
Was she lucky? Yes. But no more than a lot of celebrities/well known people are. I think to hit that level of popularity does require a lot of luck. The right people see it, right people review it, the right people read the books, a celebrity happens to receive the book for one of their kids and is seen reading it...
However if it hadn't been such an imaginative world, all of the above could have happened and it couldn't have achieved this level of popularity. So it is a mixture of brilliance and luck-like an awful lot of things.