I think he is both, good and bad, and the author also expressed this. He cheated, but before dying we see that his image showed "a single white crystal." So, in the end, he was a good person, or at least he chose to embrace his good side.
I agree that Mark was a victim in all of this. To get barely acknowledged by your son and a wife who's just as youthful as she was when they got married, would certainly hit a man on an emotional level and would surely hurt his ego. All of it did a number on him. I am, in no way, justifying Mark's actions towards JoAnn but he was a really good father. He tried everything to be close to his son but failed even when he did nothing but love him. Maybe if everything had been normal, he would have been an exceptional husband and a great father.
To be honest, I really don't remember an instant where Mark also tried to connect to Adam, he didn't seem interested, he probably used to get hurt by his son's behaviour but infidelity and cheating for me is a complete no and I really don't find Mark to be a good person on the basis of this.