Rondo has been a better PG for the Mavs than was Jameer Nelson.
That basic quote is not saying much and may or may not be true. The stats you present show things pretty equal but no stats capture the whole picture so based on the stats you presented, I would say that it is inconclusive that Rondo is even playing better than Nelson.
I suspect the bigger problem for the Mavs when Rondo is on the court is spacing. When Rondo was with the Celtics, no one covered him so it was easier for the defense to double team others or go under screens. Based on your stats, the Mavs are shooting a hair better when Rondo passes to them over when Nelson did but what about during set plays? Rondo can't make all the passes.
The Mavs so far are not better with Rondo so far. There is more to it than just Rondo but it seems silly to ignore the obvious.
In terms of being a PG, I'm going to stand by those stats. Shot creation for teammates is the primary offensive role of a point guard, and Rondo's teammates have shot with both more volume and efficiency with Rondo as PG relative to Nelson.
Floor spacing is all hand-waving unless you show some stats for it. So go find them. See if synergy has stats on what percentage of shots have been closely guarded before and after Rondo arrived, or even better with Rondo on and off the court. I assure you opponents were dropping under screens when Nelson had the ball too, and if they weren't, they should have.
I'm not saying that Rondo hasn't been the problem. I'm simply stating that there is no direct evidence presented that he has been, that can't be explained by things like the Mavs are relying on a 36 year-old to be one of their two leading scorers, or that opposing teams with film have figured out what the Mavs were doing at the beginning of the year and better planned to stop it, at least any more than floor spacing, chemistry, or whatever else is being used. I'm entirely willing to concede that Rondo is the cause of the Mavs offensive decline if presented with more than before/after numbers, since I never for a minute believed the "before" numbers to have been sustainable, even before Rondo was traded.