As in lips curled back with the teeth bared like a dog? I've only ever had one horse do that to me and he was sure as heck going to take a chunk of you if you got within range.
Sounds like a very nasty temperament but maybe I'm wrong? LOL
If there's angry signs I'd say probably yes but if it's just showing her teeth and flexing her lips up and out my mare does that when she has hay or some treats stuck in her bottom lip or if she doesn't like the taste of something. I've also heard it could be a form of testing the air but I think that's a stallion thing.
It wasnt a mean look to her and she didnt seem to be upset or getting annoyed or angry and I deffinitly did not do anything to her. Well ill pay closer attention next time, well if there is a next time, thank you!