It can be seen as insulting, or them poking fun at the people who feel the books have bad endings. It can be seen either way. In general it's not just SK, people complain about the endings of books, movies and series all the time. It just seems hard to write an ending that pleases everyone. I think during the reading or watching of a story we anticipate in our minds our own preferred ending, and that doesn't have to be at all the writer's ending.
I don't think King feels at all his endings are bad, he adressed the issue one time on Twitter and said:
"Of course for years some people have told me I don't know how to end a story. I call bullshit on that, but everyone has an opinion.
"
His endings have seldom bothered me too greatly. Sometimes the build-up of tension towards the end is better than the actual finale (Revival for example). He writes long books and you would need longer finales to do all what came before it justice, to balance it out. He doesn't always do that, but It Chapter 2 certainly does.