Hi Jintan, ya the shapes look great and composition is good as well. I think each element is well done, the only issues are the contrast and saturation between individual elements that need some work.
Zero hit a lot of good points, here is an example of some of things Xero mentioned.
Main points, keep the contrast between background elements very low- the mountains 2 colours had sharp contrast, here it was lowered to simulate atmospheric distortion. The clouds really contrasted the sky, plus they had the saturated blue outlining which would normally would be used on closer elements or the characters. Here the clouds colur was changed to not contrast the sky so much and the blue lining was removed. Lastly the saturation on the foreground elements and the player sprite was cranked to further separate the foreground from background. The sky was also made more blue per Xero's suggestion although you may have wanted a more limited palette overall.
Nice work!