He's actually saying Dumbass MARK not mort. White boys born after 1992 shouldn't try to define terms in era video games about the same year.
adj. A person identified as a loser, "buster" or a person who is easy to scam or hustle. Its origin is from that of gangsters and hitmen who would refer to their intended victim as a "mark."
"That fool was a buster-ass mark!"