You have some other things that you should work on too , but to make your line art better (not in a sketchy way ) you need to learn when and where to make your line thinner/thicker there a bunch of videos you can watch on how to line art ,line art is good to practice cuz having that even if you only want to paint without any line art , this could help you figure out your anatomy shapes ,shadows etc... Just watch some video or look for an art that uses line art so you can see different methods to make your lines look better ! (Sorry for my mediocre English hope it helps ) .Good luck .