I guess they add it because they think it looks uglier when it's over exaggerated.
Uglier cause thin lips don't really have much of an impact to make a character more unpleasant looking. Or they add it when a character has a large ego or something like that, because thicker lips is kinda a glamorous trait or a trait that beautiful people have.
In Asia, or at the very least in Japan and it's neighbors, thin lips are considered pretty, and big ones ugly. Hence, they often draw exaggerated, big and fugly lips on bad guys to make sure you hate 'em. So, for normal characters they make do with a line.
I don't really think that's the case....
I think they have normal standards for lips, they just don't like the big, swollen, cracked, exaggerated lips. Like the sausage lips on that bandit.
Well, obviously westerners don't like those kind of lips either :D but that's what I heard about asians. They like cute, so small features, including lips are well liked. I'm not an expert though, so who knows. ╮(￣▽￣)╭
Lol I'm Korean, so I know about this kind of thing, and I'm pretty sure Koreans like full lips.
I mean, if you search up some very popular kpop artists, most have full lips. For example, Jin, Jimin, Taemin, D.O. Bambam, Lisa, Tzuyu, etc.
What do manga artists have against big lips? (Granted in this case they look more like centipedes glued to his face) But really what gives?! I see so many manga bad guys nowadays with Jolie lips! Is there like one editor that everyone knows but is such a dick that artists started this passive aggressive character trend?!
Ahhh sorry not sorry that was many questions.