I can't say I am a huge fan of this idea. If I am right in Vanilla minecraft the highest number of hitpoints a mob can have, other than wither and dragon in only 20. So it is not all that necessary for it to be in the game at all.
It is however very necessary with mods like Divine RPG where the mobs have significantly increased HP.
If they do add it I would love to see an option for it to be there or not to be there.
Perhaps if a mob takes damage it could show the damage it took popping up over it's head.
or
Show it's -remaining health- over it's head
Approach 1:
Appear as numbers
This would be more understandable and quicker, but it would look too much like an RPG.
Approach 2:
Appear as hearts
Not as easily read, but would retain the "Minecraft" look.
It is however very necessary with mods like Divine RPG where the mobs have significantly increased HP.
If they do add it I would love to see an option for it to be there or not to be there.