Welcome to Basenotes!
I think generally excellent sillage refers to lots of sillage! Whether those around who smell it would find it excellent is another matter.
Personally I prefer moderate sillage. I like to be able to smell my perfume without having to stick my nose into my shirt, and for others quite near me to smell it-- but not to fill the room with it.
Poor/low sillage would then mean that the scent lies close to the skin. This tends to be true of many oil-based scents/ perfume solids.