2 thoughts that come to me after reading all the great imput.
A. There are thousands of catchers playing college baseball at all levels. If the best of the best are just over 80MPH then it is clear that the majority are throowing well under that and those that throw mid-70's would indeed be considered very good. This past MLB draft had 60 some catchers drafted. It is likely that many of those 60 some catchers throw 80+. The other couple thousand catchers that were not drafted and will never play beyond college, probably fall into different MPH groups. Those throwing 75+ should be considered good.
B. My reason for wanting to study this skill to this level of detail is to be sure that when I evaluate a catcher I am working on the correct area of weakness. Maxamizing the athletes training time on those areas that will give him the most bang for the time he invests in training.