Inappropriate? Yes. An outrage? I doubt it. FIrst, its an empty casket. Second, these are young folks who deal with death every day. Everyone has a different way of coping. And there is a such thing as gallows humor.
It was pretty stupid for anyone to take a picture and put it online, but I would hardly characterize this as an outrage in light of everything else going on in the world.
I dunno, how long is it gonna be till someone accuses the soldier at the upper left of giving the nazi salute?
Honoring the military dead is out of range for gallows humor
This brings disgrace on the honor guard
Dangerous territory and morale busting
There is a lot of room for gallows humor in war, I speak from experience. There is no place for it in the presence of a draped casket of any kind