This is a slightly edited version of the default Breeze SDDM theme shipped with KDE Plasma allowing the font to be customized.
Example using the San Francisco Display font
git clone https://github.com/avivace/breeze2-sddm-theme
sudo mv breeze2-sddm-theme /usr/share/sddm/themes/
Set the new theme in /etc/sddm.conf or in KDE Settings > Workspace Theme > Startup and Shutdown > Login Screen.
Change the font in theme.conf.user, editing the displayFont value.
Beware that SDDM is started by root, so you need to have the selected font available/installed by the root X session. SDDM will also use root X session font rendering configuration.
You can preview the login screen with:
sudo sddm-greeter --theme /usr/share/sddm/themes/breeze2-sddm-theme
There's not apparently a decent way to change the font in the default login screen theme in Plasma, a part from changing fallback/system fonts, hence this fork.
If you're here to make the SDDM look decent with higher DPI settings, you may want to edit /usr/share/sddm/scripts/Xsetup and add the line (Source):
xrandr --dpi <desired dpi, for instance 192>
Or check the DPI sections of the SDDM page on the ArchWiki.