Can mupirocin ointment usp, 2% be used on a poison ivy outbreak?

No - mupirocin is an antibacterial ointment used to treat infection. It should only be used on the recommendation of a doctor.
Talk with your pharmacist for professional advice.
First Aid treatment for Poison Ivy Rash from http://www.drugs.com/enc/poison-ivy-oak-sumac-rash.html
"Wash the skin thoroughly with soap and warm water. Because the resin enters skin quickly, try to wash it off within 30 minutes. A product called Tecnu, available in camping stores and some pharmacies, is very effective at removing the oils.
Scrub under the fingernails with a brush to prevent the resin from spreading to other parts of the body.
Wash clothing and shoes with soap and hot water. Resin can linger on them.
Promptly bathe animals to remove the oils from their fur.
Body heat and sweating can aggravate the itching. Stay cool and apply cool compresses to your skin.

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Calamine lotion and hydrocortisone cream can be applied to the skin to reduce itching and blistering. Bathing in lukewarm water with an oatmeal bath product, available in drugstores, may soothe itchy skin."