To give the shadows that nice, soft edge look set the variables filter, filter smooth, and gradient length under Light2D shadow section. Godot supports Percentage Closer Filtering (PCF), which takes multiple samples of the shadow map around a pixel and blurs them to create a smooth shadow effect. The higher the number of samples the smoother the shadow will look, but the slower it will run. But beware, it takes lots of tweaking to find the perfect result ;)