Constructing a visually good 2D game scene is all about layers.
Working from the back of the scene we can see how this scene from Ghost Dash: Scared Of The Dark was constructed. A dark background at the very back, then bringing in the moon, stars and clouds. Two levels of mountains gives a feeling of depth, followed by the main game level and finishing off with some grass in the foreground.
Breaking up your scene into layers also allows you to move each layer independently. So in the case of Ghost Dash, which is a 2D platform runner, we can scroll the mountains, clounds, the grass in the foreground and the main game area while keeping the background and moon static to give the perception of movement. Pretty basic stuff but the result looks pretty cool.
The final 2D game scene then looks like this.