I'll bet that signature layer got scaled up, which can create the aliasing. The brush creating the signature needs to be a little wider, too.
When I've done this trick, I always try to emulate look of a felt-tip pen and not a ball-point pen. It's a lot easier, and looks just as good.
I also make the animated signature a lot bigger than necessary so I have to scale it down. Things always look better scaled down than they do scaled up. And I animate the signature in its own comp, then nest it in the main comp: much easier that way.
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA