Isn't the United Earth of TOS (and by extension the system of government that characterizes the Federation) essentially based on the system of a liberal democracy as one finds with the USA?
I think the message is clearly that the US general style of government (which is shared by many other nations as well) is the most virtuous one.
TOS is strewn with this idea. On a very simple level, compare the governmental systems of the Klingons, Romulans, and any other Meanies from the show and see what they all have in common. They are all totalitarian, authoritarian, and undemocratic. In TUC, the Klingon Empire is estimated to fall in 50 years due to the economic inefficiency of its infrastructure (echoes of the USSR perhaps?).