What would lead them to believe that Mazi can provide a more consistent pass rush? The man played 30 games at Michigan and had a grand total of 0.5 sacks. Now a NT is there to plug up the run defense, so sacks aren't usually very numerous.
But I said before the draft that Dallas should not draft just the "big, fat guy" for the d-line, they needed someone in the mold of Aaron Donald (not saying there was one available to Dallas, just that Dallas needed somebody like that), somebody who could both plug the run and provide at least some pass rush.
And Smith wasn't/isn't that guy. He may yet develop into a good run stopping DT, but I'd be very surprised if he ever got more than a couple of sacks a year....