Here's a quick example to show why you can label some movies "worse" than others (even though in all honesty they're equally as bad) simply based on expectation.
Ask any comic fan "what's the worst comic book movie of all-time?" For the most part you'll get answers like "Daredevil", "Spidey 3", "Batman & Robin", "Fantastic Four", and maybe "Superman Returns".
But really there have been much worse "movies" than these. How bout "Howard the Duck"? That wasn't even a god awful movie, it was just god awful. But you won't hear many people respond to your question with that. Because first of all it's an old movie and not many people watched it, but second of all was anybody even expecting anything from that one. How about "The Shadow", "Barbwire", "Catwoman", "The Phantom", "Tank Girl", "Supergirl", or "Steel". Anyone of those movies could be labeled worse but really who had any hopes for those movies at all? Probably no one.
Meanwhile "Spidey 3", "Batman and Robin", "Daredevil", the first "Fantastic Four", "Ghost Rider", and "Superman Returns" had our expectations somewhat high going in and failed miserably. For that reason you're more likely to hear one of those as the answer to the "worst comic movie ever".
"Superman Returns" is a great example. It gets dogged on constantly and really it wasn't that bad. (I feel like Butters now "It thought it was kinda good"), but it's just that people expected so much more.