It is comparing apples to oranges. There really isn't good historical data for an inexperienced player like Exum. Every player but Schroder had college ball experience and came into the league around age 21 or older. (Lowery was a few months shy of 21 and Thomas was almost 23). Schroder played professionally in Germany for two years before coming to the NBA. So yes, these players all had higher PER, but they also had a lot more experience coming in. Dante essentially played high school, took a year off outside of the Nike Hoop Summit, and was drafted by the Jazz. So prior to this season, and inexperienced player sat out two of three years and suffered a knee injury.
Dante had a slow start his rookie year, but slowly improved month to month. He played great D, especially for a rookie, and at the end of the season, his offense started to look better as he got more aggressive. He is very tentative, probably due to age and experience. Add a knee injury to an already tentative player, and it is going to take some time to get confidence back.
The other problem with PER is it largely tracks offensive efficiency, and does not give credit for defense other than blocks and steals. Clearly this does not favor Exum at this stage compared to the above players. Dante's Defense has been good IMO. His offense is what is struggling. It looks like the coaches had him change his shot this year. It looks very mechanical (worse than before IMO). He is probably overthinking every time he shoots.
So he may not pan out, but based on his experience and the knee injury, it is still way to early to tell. I still think he will turn into a good player.