I am using the breeze sddm theme, and the login screen uses some (not exactly) arbitrary font for its text components: on one of my computers it is picking up Arial Black, on another Arial Narrow -- probably the first font that it finds. I noticed that the lock screen picks up my KDE font, so this is OK. I believe that the login screen should use some specific font, and since Noto Sans is currently the default KDE font (and most probably installed along with breeze sddm theme) it is a natural choice. Allowing the login screen to use an arbitrary font reduces visual consistency and may lead to an "ugly" login screen while the first impression always matters! Reproducible: Always
This might have been different in 2016, but nowadays I know the login screen always uses 9pt Noto Sans. Once sddm 0.19 gets released the sync option in the sddm kcm will enabled easy syncing of font you're using in Plasma (if should happen the login screen one is wrong in any way).