In the NFL of course he became great but don't forget that he had 2 full years under Bill Walsh tutelage before he really started to shine. Tom Brady is another example of a guy who became a goat in the NFL after a rather ordinary prospects heading into the NFL.
I may be a bit biased because I was acquainted with Montana when I was at ND. I was stunned, as were many people, when he turned to be the NFL stud that he became. But folks "get" what they can be at many different stages in their lives. From my perspective, he certainly wasn't "getting it" at ND.