The only way to get mobheads is by blowing up a charged creeper near the mob you want the head of. So for example if you want a creeper head you need to blow up the charged creeper near the creeper.
Some nice tips:
1) Use a shield to block the damage from the charged creeper
2) make the floor of cobble gens because then the floor won't break when the creeper explodes
Let me know if this thread helps you! :p