I am making a 2d platformer game. One of the sections is set in the sky, but I don't know how to make a good sky background, or where to find one already made. Currently, I just stuck in some blue gradients, but they look a bit plain. Is there easy way to make them look better?

Both backgrounds seem fine to me: they fit the nice cartoon style of everything else.
In a platformer, backgrounds should be plain; I suggest you remove the satellite in the night sky as a pointless distraction or move it obviously beyond the reach of the character.

I think other objectively defective elements should be improved before spending time on insufficiently cool backgrounds:
- the triangular spikes (?), which have an excessive black outline and are misaligned with the clouds;
- the cloud unit, which is too peculiar to be the only cloud type (the little clouds should be scattered variations, not a symmetrical and repetitive pattern) and badly aligned in corners;
- the spelling of "wussy".