I totally agree with this and would argue that the same thing has happened, to a less widespread degree, with Rorschach from Watchmen. I remember going to see the Watchman film at the cinema with some friends and, as we were walking to the car park discussing the film we'd just seen, one of my friends (who is savvy enough to know better) said, "Rorschach is so cool!" I agreed that he was indeed a great character, but had to remind him that actually, he's a psychotic, paranoid madman and there's nothing cool about being mentally ill.

Having said that, when I was in my teens and naturally less mature, I was definitely guilty of placing the Joker on the sort of "cool" pedestal that you mention. For teenage me it was exactly the "madness, unpredictability, and capacity for violence" that you speak of that attracted me to the character. I would routinly say things like, "the Joker's so cool" when discussing books like The Killing Joke, A Death in the Family or Arkham Asylum. Then I grew up a little bit (honest!). The trouble is, as you hint in your post, I think there are a lot of much older comic book fans that still weirdly admire the character's psychotic tendencies as some kind of warped power fantasy.