Dark humor is a coping mechanism. It’s a positive thing, I believe. As for what she is coping with - who knows?

Maybe she is struggling with depression and suicidal thoughts. Maybe it’s an awareness of the meaninglessness of life. Maybe she has had to adapt to working or living in close contact with mortality: exposure to war, working in healthcare, etc…

I think the most important part is that she is coping.

In the absence of other signs that she might be in danger or is acting out of character, I would not be concerned.