is it ok to put duct tape on iphone charger?

If you are referring to the cable (which so many people inexplicably refer to as "the charger" and you are wanting to reinforce a suspect piece of weakening cable, I'd say yes, as long as the cable is not currently displaying failings. Electrical insulation tape would be a more correct choice, but what the heck. The best reinforcement would be to stop yanking the cable and grip the connector instead.

If you are considering wrapping the hard plastic block that plugs into your electricity supply, I doubt that any good would result.

Depends on WHY you want to put duct tape on it. If it is because it is damaged, no. You should replace it.