Super Mario Bros custom tileset

[Hoeloe]

It's essentially the difference between these two lines. What you're doing is making a rough sketch of the outline (perhaps not quite as rough as in the example, but still rough), and then using that as your final line art. What you really need to do is go back over it and refine the lines. Pixel art is all about very careful and precise placement of pixels - nothing can be just done haphazardly. Because you are working in such a limited format, you need to make sure that each pixel serves a purpose, and that you get the absolute most out of each and every one. This is unlike traditional art in some sense.