He was drafted on what he could be, not on what he is right now. Also... basketball is played on both ends of the floor, so he's not the worst rookie in the history of the NBA, he's not even the worst rookie this year. PER is a stat that puts a lot of emphasis on offensive performance and struggles to adequately evaluate defensive performance.
Here's some advanced stats of the rookies this year:
click here
Dante is 42 out of 74 rookies this season in win shares. Zach Lavine is dead last.
In BPM he's 36 out of 74. In VORP he's towards the end.
In RPM(
click here), he's in the middle of the pack at 51st out of 104 all SGs in the league(I don't know why he's listed as a SG). If you list him as a PG he'd still be 52nd out of 92 PGs in the league.
In WAR, he's 46th out of 104 SGs, or 52and out of 92 PGs.
What to me is the most interesting stat is that he's the only player in our regular rotation(edit: actually along with Millsap) who's on the plus side(on +/-) when on the court:
https://www.basketball-reference.com/teams/UTA/2015/on-off/
Even Hayward is on the negative. He also has a +6.5 on/off balance. This might be explained by Quin putting him in situations to succeed and not letting him stay in the game too long when things get ugly, it also might have something to do with matching him with Hayward and Gobert a lot, but still the sample is large enough to take it seriously(about 41% of Dante's minutes are without Hayward and about 40% of his minutes are without Gobert).