I think it's a great idea to show users what parts of a project are weak (it'd help them understand how they can help), but I have the same problem with reviews on Apple's app stores, Android Market, Amazon, etc...
Some of those are handled a bit by the complaints being about previous versions, but stuff on github isn't necessarily versioned.
However, for the majority of long tail projects that might have a couple pages worth of reviews, it would be useful feedback, and the project owner could address each review individually with ease.
Another option would be to provide a history of average star ratings, similar to the 52-week activity graph. Then people would be able to see, for example, that most reviews have turned positive since the v2.5 release, etc. Tie reviews to versions and/or commits, and allow people to post another review after X weeks, so that the same person can post different reviews of different versions of the same program.