The UN exists as a forum for all governments to come together. It would have a different purpose altogether if there were some democratic bar that would have to be met to join. Also, who would decide what that bar is? There's quite a lot of grey here, and it would be very very easy for certain very powerful countries to set the bar where it was politically convenient for them rather than at some place that actually makes sense. This already happens a bit, hence why the Republic of China (aka Taiwan) is excluded, and it is almost certainly to the detriment of the people of those countries as well as their governments, imo.