But you are right. The music is so down-beat that it makes the tone of the film very much sombre.
If I remember this right, the first time Superman tries to fly - visually it's played as funny, but musically ... not so much.
I don't see what's "down-beat" about this musical sequence at all. It's full of flaring trumpets with an almost celebratory, parade-like tempo playing up the triumph of Superman learning to fly for the first time.
[yt]https://www.youtube.com/watch?v=VlINHSnUx9k[/yt]