I can’t get lips to look right, even after trying to model with half a dozen references. Every time I think it looks alright, I zoom out and I have a pouty fish face on my female human.
Practice makes perfect 
Try drawing some mouths with good old pencil and paper until you’re able to draw them well, then go back to zbrush and practice sculpting them. Using traditional materials takes the tech and UI out of the equation, which is always a good thing when you’re learning anatomy.
db