It would be pretty easy to add it so that isn't the problem. Even if it were hard to add, who cares, quality>priority when it comes to suggestions. The problem is that the OP has described whatsoever of how the mob head works. If it works the same as wit zombies, that wouldn't make sense. Mobs detect players from shorter distances if they're wearing mob heads. Maybe the villagers being scared could be a fast reaction (10 seconds) then they know its you.