Citizen Kane has long been used as a gold standard for film, and has since become a generic analogy for greatness that is often applied to other media. It's been a favorite of Neil Gaiman, when in the 80s he declared comics to be an exciting new medium because it hadn't discovered its Citizen Kane yet. Later he decided that Will Eisner's Spirit was "the Citizen Kane of comics," even though The Spirit saw print about 40 years prior to his first statement, but he's perfectly entitled to change his mind. But apparently he didn't change his mind about Citizen Kane.
But I'm not fully on board with the assumption. I think it's a landmark film, and formally inventive in a way that no film had been prior, but calling it the greatest film in history discounts some of the great films made abroad in that same era, and of course, since. Welles' acting is, as usual, hammy, even by the standards of the time, as is the acting of much of the Mercury Theater cast, most of whom were new to film acting, and the writing--well it aint Tennessee Williams. And no matter what my personal opinion is of the film, giving it this status assumes the primacy of US cinema in a way that has been typical of American film criticism of the past. And I suppose I'm just tired of the Citizen Kane reference because it's so often repeated and so often repeated reflexively, as though Citizen Kane was the unquestioned high mark of cinema just because we're told it is.
So as much as I understand that it's useful to communicate the value of comics to the uninitiated by comparing it to other media, the Citizen Kane reference is a tired one, and comics aren't aren't like other media. For one, the greater part of comics history is dominated by comics targeted at children. It was, and sometimes continues to be considered a children's medium, and it took considerable bull-headedness in the face of indifference for those early cartoonists to make great art in that environment.
Comics truly targeted at adults only started to happen in the mid sixties, and comics that attempted to be literary, Art with a capital "A", only happened in the last couple of decades. I know there were early exceptions, but this has been the general trend. So there's this uncomfortable relationship that cartoonists have had with comics history that is difficult to reconcile considering audience. Maybe Krazy Kat is Citizen Kane, or Eisner's Spirit, if we were to go with the assumption behind this analogy, but it still doesn't make sense. Only a minority of people understand comics historically, formally and as literature, and literary and film critics who are responsible for more mainstream comics criticism don't have that background. So dropping the analogy to film is useful. Comics need to be taught just as film and literature is taught, and when something like Persepolis is reviewed by the New York Times, it's important that the author of that review has real context for the work, that they're not thinking of it as formally comparable to film or prose. Or Citizen Kane.