How recreate:

Making the scoreboard:
/scoreboard objectives add mob stat.damageTaken mob
/scoreboard objectives setdisplay sidebar mob

First spawn a zombie, then:
/scoreboard players add @e[type=Zombie] mob 1

What now should happen is that you see the entity in the scoreboard, that is right. But if you damage the zombie the score for the mob doesn't go higher.
That doesn't makes sense, because the scoreboard must display how much damage the zombie has taken, but it doesn't.
That is the bug im reporting.

1.8 snapshot: Mobs were added to scoreboards, then mobs in a scoreboard with an criteria is also supposed to work.

Sorry for the wrong grammar, im dutch