Yeah, you should probably spend more time on your next drawing. The yukkuris are good but they're also the only thing identifiable without an explanation.
I think for using MS Paint, this is an absurdly clean good job. The details on the faces for the Marisas are great, and I love the little snippets you took out of the hat.
On top of GIMP or paint.net, I recommend SAI for line work. It's free as well, but it is rather more advanced compared to paint. You can customize your brush strokes and it's a lot more friendly for line work.
But yes, the environment can use a bit of work. Even just a wall + ground with a garbage can can say a lot.