Off Model is a term for when something's design is portrayed inaccurately to the original material.
Compare these two Cinderaces >>44722117>>44721811 to his official model in pic related. The heads are a little off, and their arms aren't close at all (again, Cinderace practically has no fucking hands and it's arms look like inflated balloons, whereas these two actually give him, well, fucking arms). They're closer than most fan art made of it in terms of not being *as* off model, but they are still a little off model.
My point is that by going off model, they made Cinderace look much better, because at bare minimum they made it's face less roundish and human and gave him fucking ARMS WITH HANDS. Which says a lot about how bad Ace's design actually is.