The Outlines that you can draw with Synfig are formed by a rounded brush. The brush is defined by a radius that you can modify by its brush editor widget. This tutorial explains how to make brushed outlines that would improve sustantially your artwork.
What do I need to use brushes
Well, for the moment Synfig hasn't any brush image included so you'll need to do them by your self or grab from the free resources (insert link here to those resources).
Here are some examples ...