I like natural scrolling — the inverse of how most Linux systems configure a mouse. If I want to scroll upwards, I pull down on the mouse. And vice versa.
There is a GUI for this in Lubuntu but, for whatever reason, my settings did not persist across login sessions. Which is a pain.
The answer which worked for me was:
And adding to the Input Class
Identifier "libinput pointer catchall":
Option "NaturalScrolling" "on"
This forces it on for all pointers (trackpad, mice etc.), but I could not find a way of setting it separately for the mouse.