Oh of course the word 'bad' is subjective, even in the context of film. But there's a longstanding critical Bell Curve in effect when you classify of a movie as 'bad.' Especially a movie that is (well, was) almost universally praised for it's multi-faceted greatness (at the time).
That's where my opinion that your categorizing STM as 'bad' is uneducated simply because you didn't like it. Bad in contrast to.. what? There are truly few things you can compare it to from a critical standpoint, taking emotion out of the equation.
Again, I'm not saying you can't call it bad. Just expect lots of people to not only disagree with your opinion but to provide basis for why it was/is considered a great cinematic achievement.
Why do you think Armond White is considered a contrarian? He makes a living out of flipping the Bell Curve of film criticism upside down. Doesn't mean his opinions are wrong, just means they come across as silly since you have a standard to compare them to.
Just look around the net:
What are the challenges you were presented with? How drastically different was the script you were initially presented with?
DONNER: Phenomenal. I mean, it was ridiculous. The script I read was like 400 pages that were ridiculous. They had Superman flying down looking for Lex Luthor, but he stops Telly Savalas on the street, who says, "Who loves ya baby." It was just sickening. It had no approach, no sense of reality, no sense of its own verisimilitude its own life in the reality of what Krypton was, and what Smallville was, and what the transition to Metropolis was going to be.
What were the notes you brought to re-envisioning the script?
DONNER: Oh, that's endless. As I said earlier, I wanted a sense of reality in each of the three phases of the movie. Krypton had to have its reality. Smallville had to have its own. And then Metropolis. We threw out anything that was supercilious or in any way sends up the characters, because the characters are all bigger than life to start with. Lex Luthor is bigger than life. If you compounded that bigger-than-life, there'd be no threat. He wouldn't be a worthy villain.
http://www.ign.com/articles/2001/05/01/interview-with-director-richard-donner?page=2
http://herocomplex.latimes.com/movi...-play-superman-the-way-christopher-reeve-did/
Interviews with Donner are aplenty. In almost every one (some as late as the 2000s) he talks at great length at how seriously the took the movie, and how seriously they approached the story.
Again, if that didn't resonate with you that's one thing; and it's ok. But it was there. On the page, in their intent and on the screen.