Playing on the factors mentioned in HalfSmoke's post, would most of us agree / not be surprised with the following assessment of Dunn's defense (I'm doing this without looking at the fangrapsh table):
Range - below average. He is slow. Balls hit to his left or right at the fringe of an average outfielders range will drop in because he just does not get there. I don't see any evidence that he gets great jumps to make up for his speed. The biggest plus is his height sometimes allows him to reach and make a catch when he does misjudge a ball.
Arm - below average to average. I don't see him as capable of charging to a ball in front of him, picking it up, and making a strong throw. I'll defer to others, but I don't recall strong and accurate throws home or red lights being thrown up by coaches when Dunn fields a ball.
DPR does not matter for OFs.
Errors - that we could probably look up.
I don't think there can be much of a question that a guy who hits as well as Dunn has his value limited by his defense. How much is subject to argument. I would not put too much faith in one half season of one defensive metric when there is also a consensus that 1 season defensive numbers fluctuate. Everything I've seen says UZR and other systems really need about 3 seasons of data to be meaningful, and that a sampling across systems is better than even the best system. Nevertheless, it is also interesting to note that his contract seems to value him at 2 - 3 WAR, and when you balance the past 3.5 years of performance, it looks like he is about a 2 -3 WAR player.