FWIW, I work at a University Lab school with children ages 6 months to 30 months. (And college kids but that's a different story-they can wipe their own snot!)
We use several principles in our daily care of the children and I find this to be really disrespectful of the child.

She's two. She is not capable of cleaning the snot off of her own face but she certainly deserves not to walk around with it caked on her head. The teacher should have wiped it off. I'm sorry but if it's caked on that big of an area, it wasn't that they tried to keep up with it and couldn't. They just let it go.
Believe me, I know how hard it can be to wipe all those noses so many times and wash hands every. single. time. But it can and should be done. I can see a little bit of crusties right under the nose but what you described? I've been doing this for over a decade and would never find that acceptable.
I suppose it is possible that it happened very shortly before you got in there.
I don't think I'd make a giant stink about it. It's just annoying and just a bit disrespectful to the child, IMO.
Now, I would be more upset that no one had come to inform me what was going on with my kiddo.