If Unruly Mob and another creature you control die simultaneously (perhaps because they were both attacking or blocking), Unruly Mob won't be on the battlefield as its triggered ability resolves. It can't be saved by the +1/+1 counter that would have been put on it.

If Unruly Mob and another creature you control die simultaneously (perhaps because they were both attacking or blocking), Unruly Mob won't be on the battlefield as its triggered ability resolves. It can't be saved by the +1/+1 counter that would have been put on it.