There are 4 or 5 basic styles of drawing used by Robert Crumb throughout his career. In addition to these basic styles, he has also done many drawings in other styles. But these are the basic ones, especially for comic strips:
Crumb Styles Phase 1 and Phase 2
Crumb Style Phase 2
Crumb Thick Line Style
Crumb Medium Line Style
Crumb Thin Line Style
You can see that the Thick Line and Medium Line Styles are pretty close to one another. It's all a bit subjective, but it's not just about the thickness of the lines -- it's about the whole style. Remember that!