It's really hard to say if you don't know the child at all. I've nannied for kids who really can only have a good day if they get a little crying out of their system first thing! Remember that kids have no concept of suppressing their emotions - they have to get them out, and sometimes they do so in the extremes. Especially for kids who don't get to see their parents enough because they work a lot, it's actually pretty healthy to allow them to express their sadness about their parents leaving for work.
I think an important thing to try and notice, if you can, is how the sitter responds to her. Is she trying to comfort her? Is she getting frustrated, yelling, or snapping at the child? If the sitter seems like she's staying calm when the child cries, is trying to redirect her thoughts toward something else, or is trying to soothe the child in some way, these are all good signs. If she looks tense, distant, or overly aggravated, then I'd worry.
If you feel like you want to intervene in some way, I think it would only be a good idea if you have a better sense of what's going on and what the dynamic is between the child and the sitter. Maybe next time you see them and the kid *isn't* crying, wave and say hi or good morning. Do this for a few days and maybe then see if you can stop and strike up a conversation. If you get to know them a little you might be able to get a better feel for the situation. Who knows, you might even make a friend in the process!