Here's a thought: show the rating as a 90 day rolling average with the option to view the all-time stats. Throw in additional logic to handle the cases where there are too few ratings in the last 90 days... then use the last n ratings (for some reasonable value of n), no matter how far back they go, for example.

That would seem to me to give the best of both worlds. If you release a crappy app and then later release a glorious update, you'll have to earn back your new ratings over time as old ones fall off the chart which encourages developers to release quality apps starting from day one.