I'm not sure what you mean by "pop culture" or even if it can be defined, but there's nothing unique about the "impact" of Michael Jackson. There have been man, many people throughout history who have had tremendous impact on society, in amny different fields and in many different ways. Even in music he is not unique, and this notion, to name just one, that he was more popular than Elvis Presley is simply nonsense, and I'll give you one cold hard fact to prove that. Michael Jackson never had the appeal to middle aged, mature women that Elvis Presley did, who also captured the appeal of younger women, obviously. That fact alone sets Elvis in a category apart from Michael. In fact, I was having this very conversation with a friend of mine tonight while we were in a Dunkin Donuts drive-thru, and as soon as I said it, the middle aged woman who was making our coffee, and who head me, turned toward me and agreed, unequivocally. That's not to mean that Michael Jackson wasn't a superstar in his own right...but unique? Nope, sorry.
You need to study the meaning of the word “unique”, because you didn’t get it. It doesn’t mean “number one in your opinion”, or “better than Elvis, whom you personally prefer.” It doesn’t even have to mean “number one” in anyone’s opinion. Look it up.