Plasma defaults to a fairly unsatisfying bright blue color for the screenlocker and login screen background. Almost everybody involved admits that this is sub-optimal, but we have so far been unable to achieve consensus on a better option (it's a medium-term goal of mine).
Until that happens, it becomes a distro task to ensure that users aren't exposed to this ugly blue background. The Fedora KDE spin already does this, and so do other distros. We should, too.
My recommendation is to re-use the default KDE wallpaper as the background for the lock screen and login screen.
I did this for my wife's computer when I set it up with Kubuntu 17.10 and it looks fantastic:
(Upstream task to make this this the default is tracked in https://phabricator.kde.org/T7682)