Given that in Batman '89 you rarely see any skin besides his face, I doubt many people are going to notice or care that this Joker's neck and hands are not white.
I didn't say a word about anyone
caring, so I'm not sure why you're saying this.
Memory only applies to things that are worth remembering,
Patently false. People remember whatever they remember, whether it's worth it or not.
It is laughable that you expect the general audience to remember the details of a 20 year old summer blockbuster that was more or less a film people brought their kids to. I am not old enough to say that I can't remember a film 20 years ago (I was 2), but I can contest that I have seen plenty of summer blockbuster films that were fun when I saw them but I remember very little about: I can't tell you every scene of Con-Air, The Rock, Independence Day, E.T., Jurassic Park, and many others that I say I liked, but I liked them.
I didn't say they had to remember everything about the film--they just have to live in North America. Whether it comes from the film or from somewhere else, the "general audience" knows the Joker's skin is white, by the same token that everybody knew in 1999 that Darth Vader was Luke Skywalker's father. They didn't walk into The Phantom Menace thinking "Oh, who is this guy?"
And, of course, there's no reason for them
not to remember. This "Oh, they don't remember some blockbuster!" line is silly. They remember details about a million other blockbusters, why not Batman? Batman is more culturally saturated than, say, Indiana Jones, and nobody has trouble remembering comparable details in those films. I haven't seen The Last Crusade in fifteen years, but I still remember that Indiana was the
dog's name. And, unlike Joker's skin, that was an obscure detail, unimportant to the film. The only people who might not remember are those who were very young at the time (and even that's a long shot), and even if they forgot, they eventually would have figured it out simply by virtue of living in North America, where you can find the movie playing on cable every month, along with a constant line of cartoons, comics, games, toys, and millions of people who will tell you the fact of the matter. If you know who the Joker is, you know his skin is white, in the same way that anyone who knows of Batman knows his parents were killed, and anyone who knows Darth Vader knows he's Luke's father. People who have never seen the film will know it, by the same token that I know how from Planet of the Apes ends without having ever seen it.
I remember the Batman films because I identify with the character but not everyone does; many consider the Batman films no different than any other blockbuster
Again, this is patently false. Batman is not just "another blockbuster." Batman is a cultural icon that's become a part of the cultural fabric of the entire continent--and whether a person likes, hates, or doesn't give a damn about Batman,
everyone recognizes the the saturation of the icon, and there are things that pretty much everyone knows. They don't have to
care in order to be familiar with it, just as a million people who don't care about Star Trek can still give you general information on Mr. Spock.
There will always be people who fall through the cracks, and by some miracle don't know the things that everyone else does. But they in no way make up a majority of the general audience, and I would bet they don't make up even a significant portion of it.