The Watercolor doesn't have antialiasing so if you draw a thin solid line on dry paper then the lack of pressure/wetness means that the edges of the lines don't flow out and create a smooth edge, so the thinner the line gets, the more pixelated it looks. Your example also has a lot lot of canvas texture, which does not show with wet areas because the paper has absorbed the water and spread it evenly. It's two different problems, basically, I'm not sure which is worrying you the most so I'm covering everything.

Solutions:

- Work at larger sizes. If you are seeing actual pixels, you are working too small.
- Try different canvas textures
- work on a wet background or blend the edges together into new areas of paint (this is what it is designed for)
- try using the Gloop Pen for thin linework and adjust 'Outer Blur' (it's basically the same but with different controls)




Essentially, something like the Ink Pen or the Gloop Pen is designed to use antialiasing/other settings to give you specific edges. The Watercolor is designed to blend into anything it touches, so does not always stand alone as a single line of paint very well.