In this movie all the cars are a Yugo GV, thus an unlikely 5 star rating for this little car....

In this case it would be 4 star I guess.
5 star does not mean that the car is seen a lot. It means that the car is part of the movie plot. E.g. Christine and Herbie movies that are about a car which is alive. Or Back to the future where the car travelling in time is the centre of the story.

Well, I thought, since all the cars are a Yugo GV, there are at least 15 different, possibly more. It had to be part of the movie plot, it used as somekind of a joke, since it is not very realistic that in an American movie all the cars are a Yugo GV, is it?

That's difficult to evaluate... well, I guess that it could be considered as part of movie then