Would changing the invading force to something fictional like "North Badguyistan" improve this film? Probably not. Just because this film implicitly states "Fictional America gets invaded by fictional North Korea" doesn't make it propaganda any more than any other action movie out there. Movies like "Battle: Los Angeles," "X-Men First Class," and "Cowboys vs. Aliens" all dealt with the invasion of America and the post 9/11 fear of such an invasion in the American Zeitgeist. But, it was done with metaphor and CGI muppets so no one really cared, I guess. Movies like this go back to the dawn of film. Don't get too worked up about it. Besides, I know you all have a VHS copy of "Top Gun" you can't bring yourself to throw away.
Also, propaganda or no, after "Avengers" and "Cabin in the Woods," I'd watch a flick of Chris Hemsworth reading a list of all the ways Kim Jong Il oppressed his people for 90 minutes. That dude is gold.