How to fix choppy fonts after using KDE System Settings?
Solution 1:
All font settings are stored in a hidden file in your home folder, called .fonts.conf.
If you remove it (in your home folder, press Ctrl + H, and delete .fonts.conf), and generate a new one, by running, in a terminal instance (Applications > Accessories > Terminal):
sudo dpkg-reconfigure fontconfig
You then should get the usual Linux crystal clear fonts (you actually may need to log out and log back in, but that's it)