First, just forget about dithering for now. People assume its an integral part of pixel art but it isnt. It came about as a way to create inbetween colors on old computers that couldnt display alot of colors. It can also be used for texture. Theres nothing wrong with using dithering, its just, not a requirment.
Big problem with your colors is each thing feels totally seperate. Grass is green, mountains blue, sky purple, sun orange. The image will look more cohesive if you bring all the colors into each thing. This doesnt mean that you need to take the exact blue from the mountain and put it in the grass, but maybe shifting the shadows towards blue and the highlights towards orange will help things out.