I'm not sure it is the mourning of the actual person or that they are possibly a secondary figure in someones life. At my age Carrie Fisher was someone that was a star of a movie and an action figure from something that is important to kids from that era.
Same with George Michael, I was a kid and listened to a lot of his music and watched a lot of his videos.
I think people are more mourning for an era of their life that is dying and remembering their childhoods.