Are good teachers good students?

Not necessarily. It may be quite hard to find correlation between good teachers and they being good students themselves.

Some of the very good teachers (i personally dont know about them) I have heard have been average students themselves. One Accounts/Tax teacher in Hyderabad for PE II has himself not cleared PE II. Similarly one accounts teacher cleared his CA in third attempt. 

A good teacher is one who ensures that the students assimilate the concepts. Knowledge should be priorites than the marks/ranks.