In an 8 week old? Yes, diarrhea can be serious. They can dehydrate very quickly.
You need to take her to the vet. Make sure she has a fecal done to test for parasites - worms and protozoa.
At that age, what are you feeding her? She could still benefit from some KMR (or another kitten formula) mixed into her wet food. Right now those nutrients would be very important.