Here's how the default Kubuntu Bionic install looks when you log in for the first time:
Notice the ugly full-color icon for the Show Desktop widget in the bottom-right corner? That's because the default panel height is tall enough that the widget's icon is full-color. If we just reduce the panel height a tiny bit, it becomes monochrome instead, and looks pretty:
So much better, right!?
This setting lives in ~/.config/plasmashellrc`:
[PlasmaViews][Panel 2][Horizontal1856] thickness=36
The default thickness is 36, and changing it to 34 does the trick. 30 might be even better since this change also causes the Kickoff icon to be resized too, and at a thickness of 30, the icon doesn't look inappropriately small.