I saw it. I heard it as a contraction of “you is”, which is old Amos and Andy type stereotypical black speech. And so it was, “No, nigga, 'cause you’s a bat!” With a sufficiently unvoiced “a”, you could easily have come away with “'cause you was bad”, and especially since she used a gutteral stop for the “t” in “bat”, which again, stereotypically, can make “bad” and “bat” sound like homonyms.