Yeah, that's a pretty common problem with this style. Perhaps using a very thin brush, just highlighting and shading, would sharpen things up, and I find it quite therapeutic to work with minimal options like that. I'd make a 1 or 2 pixel brush, light opacity, then sketch away with the tablet. It's a good way to add organic but controlled detail. You should have to go over a line a few times to bring it up, but this lets you shade, and have distinct highlights. Maybe introduce some sharper hair over the soft hair you have now, would look pretty cool I think.
If I'm rouging out something, I work like that, just a solid mid tone backdrop, then draw in black and white, just like using pencils.