(If you’re using Godot 3)
I’ve found that using one of the bottom two options in the spatial material in the outlines then it looks way better (I’m currently away from my PC).
I don’t remember which, but I think it’s distance fade. I found in my tests it made the outline mesh much more like a silhouette.
I’m sure there is a way to do it with shaders, but sadly I know little about shaders, so I cannot offer any help there. Maybe try searching for an OpenGL shader and see if you can translate it to Godot’s shader language? You could also try translating Unity shaders, though it may be harder than translating an OpenGL shader.
(I’m sure you’ve probably seen this, but there’s some answers explaining various ways to go about this here)