No, it's simpler than that. Millsap is strong and mobile and gives Cousins trouble defensively, especially with Howard lurking inside to help out and add physicality with Koufos unable to help out. So he's frustrated and Millsap scoring on the other end is exacerbating that.
The bigger issue to me is Cousins loafing, committing dumb fouls and otherwise not helping his team when he gets frustrated. It makes me think that if the Kings ever do make the playoffs and teams scheme better and play more physically that he'd do the same thing.
On a positive note, Tolliver continues to make the right play and is showing more and more lately.