I think a minimum of two maps to enter the rankings would be an appropriate move. The main problem here is that when a map is first ranked, it seems to get a spread of ratings that is much higher than what it ends up on after a week or two.
I would say we can improve the mapper ranking system in better ways than this, though. Allow me to make some suggestions for further discussion:
#1 We can increase the ranked map count multiplier slightly.
#2 There should be a period where new maps are not attributed to a user's mapper ranking (ie. 2 weeks after the map is ranked). This avoids rating anomalies and will allow for #4.
#3 The number of people who have favourited your maps could be considered.
#4 As was already mentioned, the playcount of maps could be considered. While this would give a higher weighting to those mappers submitting new maps, consider that being an active mapper should be considered as a huge positive and should be represented in the rankings (in my opinion at least). Given the delay mentioned above, huge spikes could be avoided to some extent, too.
If we make the mapper ranking more complex, it would be nice to see aggregate stats on the mapper ranking page - ie. total playcount/favourites figures.