Pixelart should not be downscaled due to inevitable loss of information, as you've observed, only upscaled. Look at any pixelart game and you'll see this. For example the GMS made pixelart game Hyper Light Drifter has a pixel resolution of something like 480x270, blown up to your screen size. If you need to downscale, use art with thicker color features so it resists the loss of information better.