Is pepperoni an adjective

No. Pepperoni is already a plural Word. I got this definition from Merriam Webster: Main Entry: pep·per·o·ni

Function: noun

Pronunciation: "pe-p&-'rO-ne

Etymology: Italian peperoni cayenne peppers, plural of peperone, augmentative of pepe pepper, from Latin piper -- more at PEPPER

: a highly seasoned beef and pork sausage

Pronunciation Key