I've always really liked Mack, except in season 14 when he goes completely off the rails with the Mary Robeson obsession. I just finished season 5 of my binge, and he does come close to that level of obsession with Wolfbridge--in fact, he's the one responsible for Karen being shot. But that's what reins him in, I think: the realization that his obsession almost killed Karen. He'll always be a sanctimonious hothead, but you love him for that because he wants to do what's right.
His outbursts crack me up because they are so short-lived. He reminds me a bit of Bulldog from Frasier: "ALL RIGHT, WHO STOLE MY KEYS?????? Oh, right, here they are." And his little smile that seems to put people off--the one that seems so inappropriate? I see that as a grimace because Mack doesn't know how to process what he's just heard. He's really awkward in that way. I guess I've always found him endearing. He's passionate, driven, obstinate, and sometimes really dumb, but he is a good guy and he's not as wackadoodle as literally everyone else in the show.