The text is a play on the concept of a pacifist, which is someone who opposes violence, and the irony of the confrontational challenge 'Wanna make somethin' of it, bub?', which is typically associated with aggression. The humor arises from the contradiction between the pacifist's non-violent stance and the aggressive phrasing of the challenge.
Yea , I ' m a pacifist . Wanna make somethin ' of it , bub ?