The word derives from the carnival sideshow era of the sport where a "mark" was the intended victim of a hustle or swindle. In that era, people believed that the event they were about to witness was legitimate.
1 - a person that doesn't know that the match outcomes are pre-determined
2 - a person that spends money on wrestling and watches it even knowing that it isn't real. These people like to be known as smarks but they are also marks. Just don't tell them because that usually gets them mad.
Some marks believes that everything they are see in a wrestling ring is real.