Are these lines you are creating or ones already in images you import? Are you working in the max image dimensions (1600 x 1600)?
I was looking at creating drawings from scratch. I set the image to max dimensions Gladly, like the heavenly bodies
Which He set on their courses
Through the splendour of the firmament ;
Thus, brothers, you should run your race,
As a hero going to conquest.
- Ode to Joy
Using the brush tool, I set the hardness to all the way to the right and the view to 100%. This minimized the pixellated appearance for me.