Monroe is a pretty poor defender, certainly worse than Baynes and Theis.
Monroe is a down low scorer and rebounder. It’s a trade off: offense or defense.
Both my eye test and defensive stats indicate that he is, at worst, an average defender. His defensive awareness to be in the right position and quick hands are both pretty impactful defensive abilities. His defensive rebounding is a huge help to close possessions, too.
I don't think he is that much worse than Baynes or Theis, both in the eye test and in stats. One thing that he does differently than them is he fouls WAY LESS. That can be a huge benefit to the rhythm of the game.
He's not winning the DPOY any time soon, but his defensive deficiencies are more rumor than real. Besides, if CBS can turn Sullinger into a positive defensive player (by DRPM, Drating, etc.), I'm pretty sure he can do the same thing with Monroe, who is bigger, quicker, and has a way better motor.