I wouldn't blame the lineup. He can't hit with RISP. His career avg of .221 in such situations has to be historically low. Even Dave Kingman bested that by 20 pts. You can't have a guy in the middle of your lineup hitting .221 w/RISP. Because of that he might as well hit 2nd.
It's tough to hit 40 HRs and only knock in 100 but that's been Dunn's pattern. That's another rare skill this guy's got.