I'm gonna guess that the fetish for hairlines means that she likes men with their hair up in a way that shows the front hairline.
It's common in josei to have the man glow up with a hair style that shows the at least part of the front hairline. And the pose with the man holding up his bangs, showing his front hairline is also a classic in fanservice scenes, specifically smut ones. If this is really what she means by hairline fetish then it might be common among Asian women. At least in the opinion of manga/manhwa/manhua artists.