So you start a server to be popular. I was right. :-/
Sorry, but most players won't come on a server because of their maps, but because it has the lowest ping. There are PLENTY of servers now with the almost exact same content. What if you have great maps but a very weak connexion for, I don't know, the Japanese, for example ? Why couldn't they create a local server with a better Internet access and your maps ? It's called sharing. That's the whole point of Teeworlds being an open-source and free (as in the GNU definition) software. I know, and agree, that this fact don't necessarily mean that all the maps created must be shared with the same license (it would be completely opposite to the free software way of thinking), but it still is quite sad that most players will want to be the "owners" of something.
When I created my server, I didn't do it for the purpose of being popular, as you said it. I launched it because that way I can play with my friends who live near me on a fast responding server, and maybe some random people will have a good ping on it too. And if they like it, great. If they don't, as I said there are plenty of other servers with the mods I use. And you know what ? Those servers I know are always filled, and they use the officials maps... Really, the map isn't really that important on a gameplay. Sure, there are some that can be very painful to play on, and the opposite as well, but the most important is the gameplay used and the respond time. Those are the factors that will decide if your server is "popular" or not.
And really, you should be glad that people like your maps and use them on their server. Yeah, the ones that lie and say it's their work suck, but these are just morons and I'm sure there are not that plenty. Don't pay attention to them, just be glad people use your maps. In fact, they're doing a favor to you : if they were not there, all the players who like your maps would come to you, and your server would always be completely filled, which means more work for the machine (although a Teeworlds server isn't really heavy), and more data to send to all players. That could become very laggy at some point if one of the players has a bad connexion !
I've always hated members without signatures.