Generally no, it is not normal. How far along is she? And how bad is the diarrhea? Is it frequent? Super liquidy?
Sometimes, they will have an episode of diarrhea shortly before the delivery. This might be a way for the body to ultimately not soil as much during the labor and delivery.
But otherwise, diarrhea in a pregnant cat isn't good, and may in fact merit a vet visit.
You may also want to some dextrose and mineral kitchen salt, so you can make a home made pedialyte if need be.
Here are links to some articles we have concerning cat pregnancy and birth. Hopefully they will be useful to you as you go through this process with your cat.