Mouse smoothing is a default in all Linux distributions and most desktop environments don't offer a way to turn off mouse smoothing in the GUI. The only exception I have found is XFCE4 which allows you to turn that off in the MOUSE SETTINGS. Ubuntu Unity, MATE, KDE, doesn't matter pretty much everything doesn't support this functionality including window managers for more obvious reasons so you would have to use this method of doing it.

1.) Open text editor
2.) Paste this shit in it;
Section "InputClass"
Identifier "My Mouse"
MatchIsPointer "yes"
Option "AccelerationProfile" "-1"
Option "AccelerationScheme" "none"
Option "AccelSpeed" "-1"
EndSection
3.) Save this file as 50-mouse-acceleration.conf and then open a terminal.
4.) Navigate to the text file.
5.) Type or paste;
sudo mkdir -p /etc/X11/xorg.conf.d
sudo cp ./50-mouse-acceleration.conf /etc/X11/xorg.conf.d/
6.) Reboot.