Poor, poor Superman, for so many years of existence he can boast at most two or three successful incarnations outside the comics. But what can you do? He turned out to be too strong and powerful even for the comic-book universe, and the writers simply can't come up with worthy challenges for him.
It would seem that in Superman Supernova you could tell a really complicated and controversial story about Earth's greatest hero. To present him as a man doubting his rightness, a man dreaming of leaving behind a better world than he was before he came along.
But no, instead of revealing Clark's motives, they keep shoving in disposable characters and another villainous plan. Superman fights someone, saves someone, but against the background of his gradual fading it all makes no sense. After all, the end is already known and the hero doesn't even try to resist it.
It's even funny that Lex Luthor goes a lot further over the course of the movie than Clark himself. That's not to say he becomes good, but still he realizes that the planet needs Superman.
The cartoon has a lot of absolutely nothing plot lines, which is why the main one never fully unfolds. But at the same time it is interesting, shows new characters and ends on an unexpected but positive note.