I have a very simple Plasmoid (it consists of only 1 JS file and 1 main.qml file). I've been browsing the internet for almost an eternity, but I can't figure out, how to set an icon the for the plasmoid to use in the KDE panel.
In 5 minutes I found a tutorial and a quick browse of the sources shows that you should add an "Icon" entry in your plasmoid metadata.desktop file.
See the Calculator applet as a simple example: https://cgit.kde.org/kdeplasma-addons.git/tree/applets/calculator/package/metadata.desktop#n117
That icon should then translate into those SVG icons (per theme) you have in /usr/share/icons