If someone from Iran wants to do DDoS attacks they probably know enough to change their own IP to somewhere else so your blocking Iranian IPs is basically futile.

The blanket actions you can take to reduce your attack surface will often work to stamp out low/mid effort attacks, but the more sophisticated attackers who really want to specifically target you will take the time to get around these simpler countermeasures. That doesn't make those countermeasures useless, though.