It's an interesting idea. But alas, you're not the first to have it.
It was already discussed at length in this topic:
http://tm.mania-exchange.com/threads/621/awards?page=-1
The main problem with it is that Karma is based on how well a track is suited for online. And not all tracks are meant for online. So it would be extremely biased to tracks suitable for online.
Also tracks that may be awesome but simply don't fit on a certain server may get low karma. For instance, if the server owner loaded a fullspeed track on his tech server, it would get very low karma while the same track on a fullspeed server may get very high karma.
Plus, not all servers run the Karma plugin. For instance, most league tech servers use the FAST controller which doesn't support Karma.
Those were the main arguments against it.
No system is perfect though, and adding Karma to a track page as a secondary thing (like Dedimania) would be quite interesting.
As long as it was displayed on the track page with a clear indication that this is a measure of the track's online suitability. It might not be a bad idea. It's worth reconsidering I guess.