...He has 5 powers. Unfortunately, only 1 is somewhat useful, 2 are situational, and the 2 other powers are close to useless. However, he seems to be a good scarecrow against IHulk, Brolaris, and 50% Thor dependent team. For players who player defensively, they might like him. Carbage could be a good fit for him and Iceman can be a interesting combo because he could stun opponents and reduce damage taken for his team, while spamming friendly special tiles for him to trigger his purple passive.
If the purple scaled up with covers it would be something. If this power really is supposed to be an anti-hulk and a self battery, it should have cost 3-5AP. There isn't a thor-level black battery at the 5* tier save Jessica Jones if you can find a trap, or Ghost Rider Robbie Reyes if you can get his green moving, but it's possible he could work well with Doom - I just don't think you're using his black over Doom's (or Robbie's for that matter) at that point.
I'm focusing on building teams in Pick-2 pvps around his purple powers only, and I'm ignoring the rest of his powers for now. As for his green passive, I just take it as something that gives free strike tiles. If it goes off, I treat it as a bonus. Since his purple power CDs are placed on red tiles, Thor helps to decrease the probability of them getting matched. Knull's black will also have a higher probability of converting basic colour tiles to purple tiles that creates near purple matches. As for Kitty, she will be playing catchup with him with close run of her Circuit Breaker exploding. That's why you need another ally who can churn out special tile every turn or spam a few in a single turn to decrease the probability of her replacing your next CD. At my level, 20k damage is considered a lot without booster. If only his CDs are actually repeaters... Most 5* PvPs with 1/1/1 loaners doesn't showcase the true potential of them. For Knull, his damage is already low in the first place. Even with a champed Knull, he still needs someone who can boost his damage greatly.