I just ran that command and got an icon widget for Kate
Created FolderView pointing to "Root > usr"
plasmoidviewer --applet org.kde.plasma.folder /usr* Created icon applet with "Kate"
* Created Notes applet with "Hello world" contentFolderView pointing to "Root > usr"
plasmoidviewer --* Created Notes applet org.kde.plasma.noteswith "Hello world" content