If Noosegraf Mob is no longer on the battlefield or no longer has +1/+1 counters on it as its triggered ability resolves, you can’t remove a counter from it and won’t get a Zombie token.

You can’t remove more than one +1/+1 counter from Noosegraf Mob at once to get more Zombie tokens, and you can’t choose not to remove a counter.

Noosegraf Mob’s triggered ability resolves before the spell that caused it to trigger.