Eh, season 1 had its dark moments too.

But as a general point- yes, this might be part of its role as a bildungsroman (coming of age tale). Steven has grown up some, and he realizes that there is more to life than wackerman jr. He is getting involved with his world…and there have always been implications that something dark boiled under the surface (the whole 'sudden disappearance of gem civilization on earth thing', or the implications of amethyst's birth, for example)

Even in the sillier episodes, you saw amethyst dealing with her self identity issues by going out and being a wrestler, Pearl going crazy thirsting for space, and the gems setting up a whole fake test course because they honestly have no idea what they are doing while raising him.

At the same time, while he is growing up, he is growing up fairly well adjusted, so he realizes that you need the silly stuff too. So even with peridot running about (and he is always polite to her), he does silly things like….well take anything Jamie says seriously. And you always get the feeling that steven is not fighting a war, but simply trying to keep the peace and become friends even with those that were considered enemies.

I don't think the silliness ever left, it is just that the character development and dramatic tension has grown so powerful, it is hard to ignore anymore.

Steven Universe began being deep and dark in Frybo with that Peedee speech and the disturbing Frybo costume. And that's like the 5 episode of the show, so yeah.