Brace yourself, this is gonna be long:
First, I'll start off by regurgitating what nearly everyone else just said, and say that a staff meeting is not the place to have a debate (or argument) on this issue. You said that you felt a little guilty for not standing up for your views right there, but I know some people who would flip out and start arguing with this person about the issue right there on the spot and, trust me, it doesn't make you look good. As far as Creationism being bashed with no one to defend it, I really don't think her little comment is gonna sway anyone to become an Evolutionist, if it does, then that's really pathetic (for lack of a better word).
You say most of the people you work with know you're a Christian? If that's the case, eventually she's gonna find out. If she indeed has issues with Christianity or specifically the idea of Creationism; then maybe she'll approach you about it. The important thing is to know why you
believe in Creation (whatever version you specifically believe in). I would also be prepared to make an argument that wasn't solely based on "the Bible says..."--it doesn't work with people who aren't Christians, they don't care what the Bible says.
I don't know why she felt the need to make that comment. I really don't think it was just cause the professor gave her a bad grade or anything along those lines. Imo, it's pretty clear that she's equating being a Creationist with being a "moron". Think about it, if the professor was a used car salesman, do you think she would say a "used car salesman and a moron", no, unless she had some disliking toward used car salesmen.
What I would do in your position is probably nothing. I’m not saying that’s the right thing to do, but here’s my reasoning. If she is curious about Creationism (that fact that she brought it up at all shows that she probably has semi-strong feelings about it), then she might approach you about it. If she simply views as nonsense equivalent to believing that the earth is flat, then she still might approach you about it, but more than likely you’re not gonna change her mind. My point is, that if she has doubts about her beliefs then she'll go looking for answers, if not, then there's probably nothing you can say that will change her mind.
Again, this is based on what you said about nearly all of your coworkers knowing that you’re a Christian. Eventually she’s gonna find out too.
I hope this helps. I do understand your situation. There have been instances when I kept my mouth shut, and later felt I didn’t stand up for my beliefs; however, just cause you feel that way, I don’t think it necessarily means that God is convicting you for it.
Anyways, best of luck