During the scene before the party while the Montague boys are cavorting around Sycamore Grove, Mercution pulls up in a drag costume sporting a short white afro. During the party scene, he performs his famous "Young Hearts" dance on the stairs. Lo! His afro has ballooned to inexplicable proportions!
This confused me at first too, but watch it again. That is right after Romeo takes a pill, which causes him to hallucinate. What we're seeing is his distorted, drug-induced interpretation of what is going on around him. I imagine, in reality, Mercutio's wig is the same as it always was.