Hi, I am not an artist but you could make your background image less distracting by lowering the saturation and contrast. Your player sprite should be more saturated and in a color that is not in the background. Additionally you could draw an outline around the character but that depends on the art style of your game.
If your image is so big you should consider to cut it in smaller pieces. Piskel is great for making pixel art but for some tasks you need something else.
Edit: It is also a good idea to choose a color palette and to restrict the number of colors you can use for your game/level. For your background you could choose one or two colors and for your character sprite one or two that make a contrast.
Or use a simpler background (I very much recommend that you do ). It doesn’t need to be as simple as my quick mock-up, but the main focus will be on the player and the platforms; the background should complement and not distract or, worse, make it hard to see what’s happening.
For the last image, I selected 3 colours from your background (two shades of purple/lilac and one grey). I then chose a green for the player square that has the same saturation and brightness as one of the background towers. The sky is a gradient that I made from a photo I took. It’s the one I use in my game.
The buildings could be done as a tile, and the gradient background could be a very small jpeg that is enlarged in GDevelop to the size you need.