My linework in krita is very ugly and i dont know why.
It looks very pixel-ish.
Maybe someone here knows how to solve this issue?
I have Windows 10, i dont know if this is important.

i think it's mostly because of the brush you're using. this one is a smudge brush, which makes it good for blending smooth textures, but when you're drawing only once over a sharp contrast, it'll look quite strange.

or do things look bad when you use different brushes, too?

Do you think the black brush I used Looks normal?
I want to do really smooth lines if it is possible.

PerdidoStreetStation wrote:Do you think the black brush I used Looks normal?
I want to do really smooth lines if it is possible.

Are you complaining about aliased lines, or about jagged lines.

Aliased lines can be fixed on the brush settings, modifying or selecting a different brush tip. But the black lines are not aliased.
Jagged lines can be avoided changing the Stabilizer mode.
With the "Freehand Brush Toll" selected, look for the "Tool Options" docker.
Change the dropdown from "No Smoothing" or "Basic Smoothing" to "Stabilizer".
Mark the "Finish line" and "Stabilize sensors" checkboxes.
Play with the settings until you are confortable. The most important setting is the "Distance".