That I would believe. Soldiers and policemen do it a lot -- it helps distance them from the shit that is going on. (Apparently morticians are the worst, so my embalming friend tells me). Gallows humour.
Doctors do it too.
But this ties in with creating a character and situation where it is believable. Someone unused to blood and gore who just saw a horrible accident probably wouldn't.
I second the statement that a lot of what I buy depends on how much I like the story. Also, authors earn a certain amount of credibility as I read. if they get some of the small things I'm knowledgeable about correct, then I trust them more something seems off but I'm not completely sure.