Dom is an excellent athlete who is a very good defender with below average discipline. There are things Dom can do many others can't. Calling him a poor defender is ludicrous.
There are aspects of team defense that Dom struggles in and you can't down play how big a factor having below average discipline impacts the team defense. Due to his length and athleticism, he's a great disruptor when played on top of the key. However, he struggles to keep his man in front of him which breaks down the integrity of any defense. He struggles in his spacing, not being in position to help on dribble drives. He struggles in his court awareness, eyes on ball and man. He struggles with his rebounding, not boxing out. He struggles on his rotations, not helping the helper or being late. He struggles in his recovery, not getting back out to his man on the perimeter. Dom can do many things others can't, but if he doesn't actually do it, does it really matter? Quantify what Dom does well that makes him a very good team defender.