I've seen this type of facial expression in a few animes, but as far as I can tell there is no official tag for it, so I'm calling it m-mouth. Because it's sort of shaped like an M.
Works well for embarrassed / surprised / annoyed faces. Just combine with an appropriate tag, and specify either open or closed mouth.
It does tend to produce the same few hairstyles if left to itself, but you'll most likely be specifying your own hair or using a character lora anyway, so who cares. Seems to work well with character loras.
Trigger word is supposed to be m-mouth, but it doesn't do anything.
Update
I see now that the Lora, especially the Illustrious version, often produces a couple of sweat drops on the face, so you might want to add "sweat" to your negative prompt if you don't like this.