There are so many different little issues with Zinger. First and foremost, he's proven to be just as injury-riddled with the Mavs as he was with the Knickerbockers, which is okay when you're a rookie but not when you're being payed max money to be the second star paired with your yyoung MVP candidate. Second, personality-wise, Porzingis is just not awesome, giving poor effort at times and displaying poor basketball IQ at other times. Also the man is just sawft in the paint. Third, Porzingis really isn't a good pick and roll player which is a huge concern when Luka is your lead ball handler. Part of this might be Carlisle refusing to run pick and roll actions for them as much as he should but Zinger spends more time parked off ball away from the the action or worse in Luka's way on a concerning amount of possessions.
Zinger got the New York media boost to the point where he was elevated to superstar status when in reality he might just be high-usage Raef Lafrentz. The problem now is that he's been so injured and bad this season and his contract is so expensive that if you try to move him, you're probably not going to get anywhere near the amount of assets back for him as what you gave up to the Knicks in the first place.