I know it doesn't work this way, but Barnes is having a not great, but respectable season if you eliminate the two Warriors games. Those were by far his worst two games, and I suspect that it is a matchup that is bad for him. If you eliminate the two Warriors games he is averaging 12/6/2.5, which is not terrible.
The interesting thing is Barnes' shooting splits. On two point shots he is shooting
Season: 50%- 23/46 (2.55/5.1 per game).
Career- 49% (3.8/7.7 per game)
Kings Career- 52% (3.45/6.65 per game)
So he is right on his normal average. The interesting thing is the volume. 2.5 less shots per game from his career average and 1.5 less shots per game from his Kings average. So he is shooting the same percentage from two, just on less volume.
From three point range, the problem is opposite. He is shooting the same amount of threes as his career average (but less than his Kings average), but making WAY less.
Season-- 5/30- 16% (.55/3.3 per game)
Career- 38% (1.3/3.5 per game)
Kings Career- 39% (1.7/4.3 per game)
If you could get him to his normal level of aggressiveness inside, while increasing his efficiency to a more normal level from outside, you would add an extra two and three per game, and you would be talking about Barnes averaging around 16/6/2.5 (i.e. right around his career averages).
That said, I have no idea why he is less aggressive inside and less accurate outside, but he isn't as far off as the stats would suggest, at least offensively.