Hoot.

Hi Thanks for the ask!

Is this the red eyeliner you mean? I am not a player of Genshin Impact and this is what google told me…

I think since 2018 red eyeliner and red eyeshadow are kinda popular in China. This kind of bright red is thought to be sexy and fierce (I think). 

Traditionally there is something called 桃花妆/taohuazhuang (plum blossom make up) but it is more like a blush makeup that continue until the eyes. This kind of makeup is most popular in Tag dynasty.

The make up for the top picture was more traditionally styled than the bottom picture. Both could be called  桃花妆/plum blossom make up.

Ziseviolet has a makeup tag that features a lot of traditional make up, including the plum blossom make up. 

In the case of the Genshin Impact characters, though, their red eyeliner is probably just a stylistic/aesthetic choice by the designers. You can see this type of makeup in Chinese costume dramas too, like on Wen Kexing from Word of Honor below. Like @fouryearsofshades​ mentioned, this makeup (in cdramas) usually denotes “fierceness” in a character: