Gnome-Screenshot is the default tool to take screenshots on Ubuntu and Mint and brings a great deal of functionality. While it also has the ability to exclude the mouse pointer for screenshots if you use the graphical interface, you won't find this feature if you want to take them from the command line - the only option to exclude them there is to set the option as default which is possible with dconf-editor.
If dconf-editor isn't installed on your system yet, install it by entering the following into a terminal:
sudo apt-get install dconf-tools
Afterwards you can change the default behaviour of pointer inclusion with
dconf write /org/gnome/gnome-screenshot/include-pointer false
If you want to revert things, just set it to true again:
dconf write /org/gnome/gnome-screenshot/include-pointer true
Next time you open the GUI or take a screenshot via PrtScr or the terminal, the mouse pointer won't be included on the screenshot.