Does XFCE/Xubuntu have a system monitor panel applet?

Gnome has a system monitor panel applet that can be seen even when all your windows are maximized:

Is there anything similar for Xubuntu / XFCE?

You can install Multiload-ng or XFCE4-hardware-monitor-plugin as all-in-one solution. Also some separate solutions are in the xfce4-goodies package.

Actually, after building Multiload-ng correctly on XFCE desktop environment, there will be three apps from this package:

• Standalone app (has its own window, not embedded in any panel)
• Panel plugin (version of your desktop environment is automatically detected during build)
• Standard system tray app

So, you can use Multiload-ng-systray version even if your panel is not supported or you can not resolve some panel-specific dependencies (in such case remove --disable-autostart option during build).

Installing Multiload-ng

Update your OS:

sudo apt update
sudo apt upgrade

Install dependencies (depends on your system, check for warnings and errors during build). Tested on:

• Linux Mint XFCE Edition 20.1 x86_64 (XFCE 4.14)

sudo apt install git autotools-dev automake libtool intltool libgtk2.0-dev xfce4-panel-dev

• Linux Mint XFCE Edition 20.2 x86_64 (XFCE 4.16)

sudo apt install git autotools-dev automake libtool intltool libgtk2.0-dev libxfce4panel-2.0-dev

Clone source code:

git clone https://github.com/udda/multiload-ng

Build (check for warnings and errors):

cd ~/multiload-ng
./autogen.sh
./configure --disable-autostart
make

Install:

sudo make install

Installing XFCE4-hardware-monitor-plugin

Update your OS:

sudo apt update
sudo apt upgrade

Install dependencies (depends on your system, check for warnings and errors during build). Tested on:

• Linux Mint XFCE Edition 20.1 x86_64 (XFCE 4.14)

sudo apt install git libgtkmm-2.4-dev libgnomecanvasmm-2.6-dev libgtop2-dev libxfce4ui-1-dev build-essential libgtk2.0-dev xfce4-panel-dev intltool

Clone source code:

git clone https://git.xfce.org/archive/xfce4-hardware-monitor-plugin/

Build (check for warnings and errors):

cd ~/xfce4-hardware-monitor-plugin/
./autogen.sh
./configure --prefix=/usr
make all

Install:

sudo make install

The screenshot below also includes xfce4-weather-plugin and xfce4-datetime-plugin from official repo.

Try using Multiload-ng, it is a bit old but it works on Xubuntu 20.04.
You will need to compile it yourself tho.

Sure there are.

System Load Monitor, CPU Graph and Network Monitor are panel plugins of Xfce. They can be installed as dependencies by installing xfce4-goodies package. And you add the panel items by first right-clicking on panel or by using Panel dialog.