This would improve consistency.
Created attachment 145417 [details] Icons in SDDM
Created attachment 145418 [details] Icons in system tray
For the battery icon, there is no reason to use an overlay purely for the same of consistency, since the overlay is a compromise driven by a lack of space in the System Tray, and no such constraint is present on the login screen. As for the virtual keyboard, it's already using the same icon; the icon you're looking at in your System Tray is for the physical keyvoard, not the virtual one--hence the little wire coming out the back.
Understood. I think the battery icon doesn't need to have the overlay, it just needs to be the same as the systray one. Also, maybe the icons could be a little bigger and vertically aligned (the virtual keyboard isn't) and this would also improve the battery icon crispiness, because currently it looks blurred.