Hey, I touched up a couple of things. I'll see if I can explain my thought process

First is that the mountains were very "noisey", meaning they had a lot of scattered pixels mixing together. And that contrasted a lot with the rest of the piece. I think it still does a bit in my edit, but the first thing I did was use cleaner, larger strokes of color. I also replaced one of the colors so that there are only 3 colors.
Additionally, the mountains you had look very isolated. Mountains tend to group up together and make mountain ranges.
I think having the mountains both isolated and small make them look more like a pile of dirt than a mountain.
On that same point I think the shape is important. Mountains rise up from the ground, so they should feed continuously back into it

And then the last thing I did with the mountains is I added some anti-aliasing. You can see the mountains in the middle have be AA'd, whereas the ones on the sides have not. I did that by taking a midtone pixel between the cloud color and the dirt color and just added it to the sides.
Zoom in to see what I mean.
Okay, and then I touched up the clouds and I did a couple of things:
First is I made everything the same hue.
Next I thought that clouds were kind of going off in all directions and i thought they might look better if they were all horizontal. You did that well with the mid-tones, so I just did it more and also with the white.
last bit is a tip I was given a bit ago, which is that in general, white it too bright. consider how blocky it makes the non-AA'd mountains look. It helps to use a really bright off-white color rather than white.
I hope you like my edit and I hope it helps you.