A joke because it's the opinion of the general population (users) instead of some random critics? If anything, I'd say it's superior to the Rotten Tomatoes score (Tomatometer).
When people rate movies, they don't rate them according to their opinion on other movies. They rate according to their overall experience and enjoyment of that particular movie. You can't compare movies 1:1 based solely on their rating. Both The Godfather pt 2 and The Dark Knight share a rating of 9.0 but they are completely different movies.
Furthermore, it's become more popular to downplay certain movies just for its genre. Die Hard is just a simple action movie but a fantastic one at that. And yet, it's better rated than idk, 12 years a Slave which won the Oscar for best picture. Movies don't have less merit just because they are blockbusters.