A Rating won't be that good will it? It's not indicative of the map, but the subjective experience that player had at that moment. For example there are a lot of people who seem to hate Under, yet it is voted for an aweful lot by the community. So by default you have a bunch of players who hate the map flow and will rate against it, while another group might be influenced by how well the game was played. I prefer close battles where my opponents are about equal in power and skill, yet as a rule a lot of people will go "GG!" the moment they can absolutely crush their opponents despite them beating up an outnumbered bunch of newbs. And even that loss can cause people to rate it negatively even though there might be nothing wrong with the map.
I think the forums are a better place to discuss the maps. Perhaps you could make a weekly/monthly discussion about a single map, and also advertise that in-game so players are encouraged to come here and talk about it. That way you get a more clear idea of the pro's and con's of each map as you take in the entire experience, not just that one game where people weren't having fun. For example: Besides that it's picked a lot, what is the reason there is a hard-core anti-Under group that get's outvoted so much? Is it the map flow? Is one faction much more likely to win? Perhaps add the win/loss ratio of GDI/NOD in the thread if you have those numbers.