This problem is compounded by the fact that he has practically no lines of dialogue in Man of Steel. So that's two movies in a row where he says very little.
Plus, he knows how much he'll be in Justice League. He may not say much there either.
That's the problem. We don't really know this version of Superman. Many of the complaints are just people taking what little we do see him say and do, contrasting that with their personal idea of Superman, and flipping the fuck out when the two don't align.
They really need to give this Superman some agency. I'd argue that's more important than lightening him up. Even if you want Supes to be more of downer (or just more introspective), you have to make us understand him and his point of view.