...and now they don't have faces! Why?

It seems to depend on how individualized the mobs are supposed to be; the monochrome NPCs _with_ faces seem to be the ones with lines.

Mouths don't count. They've always had mouths any time they need to speak. Recently though, they've started getting eyes too, but only in some random instances. The ones in this strip talk but don't get eyes while some of the ones in this strip don't talk but do get eyes. And the idols don't even seem to notice! Are they supposed to have eyes or not?

Nerniply said:

Mouths don't count. They've always had mouths any time they need to speak. Recently though, they've started getting eyes too, but only in some random instances. The ones in this strip talk but don't get eyes while some of the ones in this strip don't talk but do get eyes. And the idols don't even seem to notice! Are they supposed to have eyes or not?

Looking at your examples, it looks to me like they get eyes if they're directly interacting with the idols.

Doing it this way enforces that the interaction with the kids in post #4263622 is more personal, while the people in this strip and post #4249230 are just random bystanders. They don't need eyes because their identities don't matter, because they're just anonymous people on the street whose faces you won't even remember tomorrow. The girls in post #4227030 look to be the idol's school friends e.g. people she knows.

The idols don't notice because it's a stylistic thing, they obviously have faces in-universe.