Earlier quoted context omitted.
Someone should dig out the relevant source code for you to fix it. I'm sure a good PR will be well received :-)
The UI code for the system tray can be found here: https://invent.kde.org/plasma/plasma-workspace/-/tree/master... and the HiddenItemsView.qml is the part show on the screenshot. MR are always welcome.
Looks like the window should default smaller to make the sizes reasonable, or whatever decides the columns/row counts needs some tweaking.