https://en.m.wikipedia.org/wiki/List_of_best-selling_fiction_authors
JKR is massive in terms of sales and there is a good argument she is the best selling chikdens author ever as well as allowing a hugely successful film franchise.
She has a little way to go to beat Shakepeare but her has had a 400 year head start. I think the point going is that even if JKR could match Shakepeare in sales it doesn't make her Shakepeare. (She is a hel l of lot more successful tha many classic authors e.g. Austen from a commercial perspective).
I think therefore that will be some that wish to have a bit of a dig at HP and his followers as they feel there is a danger of JKR being artificially inflated in terms of literary gravitas.
JKR is also very much a femonist and took risks in advocating GC views (quite courageously). I think this makes here a mandate character in society as a whole and I think there will always be those that take a pop.