DeMar's D isn't bad at all.
He has trouble with some of the craftier veterans... That's to be expected.
I'd like to see him get some more steals (by jumping into the passing lanes). A perimeter player with great athleticism can do that. The steals a perimeter player gets turn into points more often than not.
If memory serves me right, he was getting a bunch of steals after the all star break.
Give credit where credit is due - he has improved and if he stays aware and aggresive on D, then with a little more experience he will make a name for himself on that end of the court.