Porzingis play's like he's a 6'4" guard trapped in a 7'3" center's body. The Mavs offense would be a lot more dynamic if Carlisle forced him to work the midrange more often. That's why 7 footers like Dirk and Durant were/are so deadly. Both had/have the ability to hit the 3-pointer but the bulk of their scoring was/is done in post and midrange.
If Porzingis ever decided to get serious about working on his midrange game, I could easily see him return to being an All-Star caliber player. But as long as he's content settling for long range jumpers, he'll be nothing more than a high end role player.