I agree that there is some problem with this. It's true that low quality custom maps often discourage people from playing custom maps at all, which is a bad thing in my eyes. However, I believe there is not much we can do about it. People are free to host their own servers with the maps they want on them (even if they are really bad) and this is not something that should change.
There was a project called "Qualitee" or similar some time ago which worked on exactly this problem. People could hand in their maps here on the forum and if they met certain criteria (i.e. some quality standards, like no graphic bugs, no hidden paths etc. were met) the map got the tag "QT" (I believe it was like this, don't be mad if I am mistaken). Servers which only hosted "Qualitee" maps were allowed to have the tag "QT" in the server name.
I think it would be a good idea to continue this project, but officially. We could implement something like a "quality maps list" that is automatically by the client retrieved from some server (e.g. the master server) and then the client will display a "QT" sign (maybe an icon or whatever...) in the server browser. This way you can at least find quality maps already in the server browser. People could apply for their maps to be listed on that "quality maps list" here on the forum (or somewhere else) where it will be reviewed by certain authorized people, (like moderators, official map developers etc.) who then judge if the map will be put in the list and if they accept the map, they will put the map on the list.
Please let me know what you think about this idea.