First time I ever saw my father cry was when Lennon died. And he wasn't/isn't even a particular Beatles fan - mum is and she was also very upset but I kinda expected it from her.
What my dad felt was shock and also saddened as lennon was so young.
As a child at the time I didn't really understand.
When Freddie Mercury died again even though I wasn't especially a queen fan I liked their music and respected and admired Freddie and his talent, plus it was for me the first aids death that felt so close to home.
My mum had been upset by Rock Hudson's death but as he wasn't a celebrity I'd grown up with or related to it didn't affect me the same, but Freddie's death really did, but I suspect partly selfishly because I was 17 and in my first sexual relationship it was an uncomfortable reminder of the reality of hiv and aids which at that time was still very much a death sentence, I don't think those who reached age of consent 10+ years later when treatments for hiv and aids were massively improving and the public health campaigns changed very much in tone "get" how it was at that time.
It was also a shock as this was pre internet days and it really had been a closely guarded secret, iirc even though Freddie was known to be gay when he appeared visibly ill a lot of people were thinking cancer rather than aids.
Also the timing of the announcement and then the death.
But the one that hit me hardest was George Michael. I've been a fan since I was 10/11, bought everything he ever did and saw him live, also because it was relatively unexpected. Yes he'd been ill but appeared to be on the mend and taking care of himself plus Christmas unfortunately in my family is a time of several bereavements anyway.
I'm generally pretty hard faced but I was a wreck upon hearing the news and for some days after and even after that found it incredibly hard to listen to his music for months, especially the more melancholy songs.
Couple months ago my favourite radio station paid tribute to the album "faith" that was the first time I'd listened to a full album since he passed.
His music was very much the soundtrack to my life and certain songs are very much linked to certain times and events in my life.
I don't think your child is unusual at all in their reaction. Particularly with teen hormones running amok. And quite honestly the time of year too when the dark nights etc can make all our moods a bit low.
Sounds like you have a caring teen there.