Bollocks! I haven't found anything more delightful and interesting in his books than an apeparence by "Clive Cussler".
I am sure that is true, but that is more an issue with the author and quality of his work than his resorting to cheap tricks.
Stephen King makes appearences in his books and he's far from egotistical and whatever else you mentioned. In fact he's the world's bestselling author. It's something a lot of authors do. It's not like they make their characters out to be brilliant or anything.
He's not?
He may be a best selling author, but that really does not say he is good quality, just that he sells a lot of books. Dan Brown is currently the worlds best selling author, but what he rights is contrived trite. They appeal to a reader type, like most best sellers. Does not make them quality. Seriously, read anything by Charles Dickens if you want to know how utter drivel becomes classics.
Was Hitchcock full of himself and egotistical to make cameos in all his films?
Yes, totally. And he knew it.