An editor with mediocre skills can oftentimes be more successful than an editor with a real gift for the art. I know this because I have met and worked with some incredibly gifted editors who are also, unfortunately, poor communicators (or just assholes) with their authors, and I have also worked with some average editors — not particularly gifted or brilliant, but possessing decent skills — who make it their job to also remain highly supportive of their authors and communicate well with them.
It doesn’t matter how brilliant an editor is if he or she isolates the author, because an isolated author will not be receptive to that brilliant editor’s advice. An editor is only as good as the percentage of influence he or she has with the author, so a brilliant editor who manages to only get about 20% of his or her recommended changes made by the author can be beaten by a mediocre editor who — due to an established, trusting relationship with the author — manages to get 60% or more of the recommended changes he or she made by the author.
Net result: the mediocre editor’s book will be better edited and will therefore be a better book than the gifted editor’s book.