It's certainly better to have a mild problem, for a number of reasons. First, if your problem is mild, it's likely that the rest of your face is more balanced, since your overall facial development was probably more normal. So your starting point is already higher. Second, smaller jaw movements are much less likely to look unnatural than larger ones. Third, a surgeon can much more accurately predict the aesthetic effects (particularly regarding soft tissue) of a small jaw movement vs. a large one. This would allow him to come up with more ideal movements and better simulations. And of course, there are non-aesthetic reasons that someone with a mild case should do better (lower risk of numbness, for one).
In short, the final outcome should be better for people with mild problems, even if the total improvement is smaller.
