For inserting himself as a character a bit I didn't think it was poorly done. He took a big risk in even doing it and I think it comes out fine. What bothers me isn't the meta nature as much as how he shits on his own work towards the end of the series. Oh, Insomnia is "thin" you say. It's a fictional story with approximations of people and themes filtered through the vessel of Stephen King from the turtle or whatever. It's not "true" in the same way the Dark Tower series is. All these connections between the books aren't literal in the sense we all thought they were. They're metaphorical or approximate. They're double fiction.
I'm more bothered that King made up a fictional organization that combs through his works looking for references to the Dark Tower. Yes, your fans are invested in the thing you are doing. Why are you making fun of them? And what difference does it make anyway given how you wrap all this up? Is every single character we ever see ever also in some kind of purgatory? What the fuck are we even doing here.