When it comes to showing clothing, that would not only requiring the ghosts to have an inventory, as mentioned, but would also require that inventory to be populated with ghostly copies of the clothes that are lying in a heap with the corpse. Those ghostly clothes would then also have to cease to exist together with the ghost, would be immune to efforts to mark them for dropping or dumping, and be blocked from showing up in the fortress inventory. Thus, a whole new set of rules would have to be coded in for a visual effect that has no effect in the game as such. So, in my view, it's a nice idea, but too much work required to be worth it.
I don't think that's needed, when a creature that can become a ghost dies on map have the game make a "stamp" of their current sprite cloths, gear and all.
The stamp would just be a sprite assembly instructions for that ghost if that creature does raise as a ghost use the stamp for its sprite, the ghosts sprite's look and appearance will be "set" at death and never change after death.
Then you're only telling the game to assemble ghost sprites differently but not changing anything else, is it worth it? IMHO yes, but I also want paper doll face portraits for adventure mode conversations, to me the only real question is can it be done not should it be done.