I wouldn't say that Superman was turned into a humdrum loser in this one, but that's mostly because I don't think they did much with his character at all. Clark had very little dialogue, Supes had even less, and they were far more concerned with making him into a Christ figure than making him into a round character.
I think, having thought about it a little more, that perhaps what bugged me was Supe's lack of purpose. In the film he seemed to be very reactive. He didn't have a goal he was working towards, aside from the one that Lex gave him. Once Lex appeared he had a goal, stop Lex. But he never seemed to have a clear purpose beyond that. "Stop random people from dying" is a purpose I suppose, but it's a nebulous and, in the end, unattainable one.
Christ on the other hand had a
clear purpose. Show people the
one true path to God. This superman was a muddled mess who didn't know what to do with himself. And trying to make a generic Christ figure out of a superman who is thrashing about in a search for meaning seems self-defeating.
Deliberately leaving out the "...American way." part of the Truth and Justice quote is just a symptom of the whole problem with relativism. Truth and Justice are ideal goals, but how do you get there? Can you get there with any old "way"? I don't think so. The world is chock full of examples of "way"s that don't cut it. The American way used to be openly acknowledged as the best way to get to Truth and Justice. Nowadays though it's popular to compare our government unfavorably with say, the government in Iran.
This Superman film tried to turn a conflicted character who didn't know his place in the universe into a Christ figure. I would have enjoyed a version of this film that focused on Supe's character rather than trying so hard to stuff him into a wrong-shaped container.