I don't know of any program or plugin that monitors your peripheral device's battery levels. The power management in XFCE is for the computer system itself.
No it's not only for the system itself. Have you tried it? Would you like to bet a million on it?
It also monitors the battery levels of wireless devices such as Mice/Trackballs, Keyboards and even connected devices such as an iPhone as shown in the following images.
Manjaro Xfce 0.8.10 Power Manager System Tray Icon showing Battery Level of my Logitech Trackball
If you right click on the Icon and select Power Information
the following information is displayed. This is also from the Manjaro Xfce 0.8.10 environment.
As you can see in the next 3 images it's broken in Calculate Linux Xfce 15 which is using later versions of software.
Some Google searching confirms that the Power Management interface is broken in later versions of Xfce.
With Cinnamon installed for Calculate Linux 15, the Power Manager icon, in the Panel, displays the Battery Level for my Trackball.