I don't think the judges are getting on to D.L. just to be mean. They're there to score the contestants and give them feedback on how they can improve. It doesn't help the celebrities improve if the judges give them a pat on the back each week just for showing up. The criticism might be hurtful to the celebrity but the judges are also as open and honest in praising celebrities when they do show improvement.
D.L. has a lot to work on. If he keeps practicing and still isn't getting better, at least when he finally does get eliminated he can hold his head up, knowing he did the best he could. He and his fans shouldn't hold it against the judges if he's not the best of the celebrity dancers, despite his efforts.