fitting mud flaps to your car might be easier and safer or just accepting that bikes get dirty.
Obscuring what remaining view you have from your rear window is not particularly safe from a driving point of view (my towbar mounted bike rack takes 4 bikes and you can still see through the rear window, although it is not fantastic, it is better than nothing).
You also have the issues that I don't think I know of a single bike cover that is designed to go over a bike rack of any type and we have tried many because we actually use out towbar mounted bike rack to store our 2 mountain bikes at home (outside) and finding a single bike cover that covers both bikes and the rack (otherwise you have to deal with 2 arms that get in the way that clamp the bikes onto the rack) has been exceptionally hard - they are designed for 1 bike only. It often blows off in the wind as well, and we have had to make sure that the one we got was breathable because on the cheaper plastic ones moisture built up on the insides and caused saddles and bars/bar tape to go mouldy.