Actually the joke is the last comment "А зря." Actually the plot summary is expected to the "plot summary" i.e. the short explanation of what you can expect from the movie. While the last sentence is a personal comment of the plot summary author about the character behavior (not about the movie, the plot, the picture or acting). The author express his thought that "She would better not been doing this" in an extremely non formal way which is not expected to be printed in a paper.