bad quality of a video

[cyberriman]

with every video i wanted to see it is always a bad pixeled quality.my inet is good,and if i try see video just with mpv "link",it works good.
i tried this command

set playerargs --ytdl-format=bestvideo[height<=?1080]+bestaudio/best
but it didn't help too.
what is the problem and how to fix it?