The xinitrc won't be sourced by SDDM, move that stuff (MINUS the "exec dwm"!) to ~/.xprofile
Edit: though it will then also be sourced by plasma.
In doubt create a /usr/local/bin/dwm.sh wrapper and run that instead.
In /usr/share/xsessions/ directory I created a dwm.desktop file:
[Desktop Entry]
Encoding=UTF-8
Name=DWM
Comment=Log in using the Dynamic Window Manager
Exec=/usr/local/bin/dwm
Icon=/usr/local/bin/dwm.png
TryExec=/usr/local/bin/dwm
Type=XSession
and in my ~/.xinitrc file I have added these lines:
# DWM Status
dwmstatus 2>&1 >/dev/null &
# Keyboard Layout
setxkbmap -layout us,gr -option grp:alt_caps_toggle &
# Compositor
picom -f &
#Add Wallpaper
nitrogen --restore &
#Execute DWM
exec dwm