That is needed only on non Qt Desktop … Like Gnome, XFCE, Cinnamon, Mate … etc. KDE Plasma has the Settings > Appearance to deal with Qt Widgets and Style …
If by chance you are running wayland session then you can make use of
export GDK_DPI_SCALE=1
export QT_AUTO_SCREEN_SCALE_FACTOR=1
export MOZ_ENABLE_WAYLAND=1
export XDG_SESSION_TYPE=wayland
export PLASMA_USE_QT_SCALING=1
either in environment variable or your .profile or .bash_profile
For SDDM use HiDPI login screen - #2 by cscs
Also, according to your screenshot, you are using the legacy SDDM theme, please either use the Breath one or Breeze one.