I don't have a perfect solution, but there exists a setting in KDE to show a line bewteen the different components of the kicker panel.
Here is a way to set it (I don't know the exact words in English):
Right click on the panel, select configure in the menu. Then select looks (or aspect)->advanced option. There will be a new pop-up window, and you have something like "Miniprogram indicator" with the options: visible, estompated, hidden. Play with them and see if visible/estompated satisfy you.