1) Someone who adds heaps and heaps and heaps of crap to a spot just to get a medal and usually stops adding as soon as they get the medal that they want.
2) Someone who adds a small amount of content to a spot and then asks a number of their フレンズ to rate their content so that they can get a medal faster and with less work.
3) A combination of both 1 & 2.
There are probably other definitions that other people have too.
Medal whores are basically people who 提出する crappy content to a spot and ask many (Emphasis on the "many") people to 'like' it, so they will receive a medal for that spot without doing very much at all. And then they do this repeatedly on other spots. [Correct me if I'm wrong ^^']