A man shooting an elephant out of a gun is funny! Or, it could be, if it were that kind of movie, and the elephant had a goofy expression on his face, but the man firing the gun isn't surprised at all, or...

Anyway, my point is, if it's THAT KIND OF MOVIE, then it could work. Then realism doesn't matter. But JUNO is a movie striving to be authentic. It isn't HEATHERS, which is of course not realistic, but is a million times more authentic in what it has to say about being a high school aged girl (not to mention funnier and more interesting). So, you're right, who cares about realism? Maybe it doesn't really matter that there's no way Juno would know who Soupy Sales is. But it DOES matter that it doesn't say anything interesting or true.