Is it okay to mulch iris with wood mulch? If not, what is a good option?
Putting fresh wood chips/mulch in and around garden plants is never a good idea as it will quickly burn the plants. That said, if they've been sitting for some time, it should be ok to add them to the garden as mulch, though it would likely be better to compost them first.
When mulching any plants, including iris, it is best to apply it at least 3-4 inches deep, leaving at least an inch or so around the plant itself to avoid issues with rot or fungus. For more information on using mulch, this article will help: https://www.gardeningknowhow.com/garden-how-to/info/whats-the-best-mulch-for-my-garden.htm