Options / Preferences / View Options / Mouse View has checkboxes "Invert X" and "Invert Y".
I wish they did work for Mouse View, as the box around the checkboxes indicates.
They seem to invert only the mouse in guns, but do not have any impact whatsoever on the mouse view. It's a bit strange that the mouse acts on the gun without being configured to do so in the controller mapping. Simply by enabling mouse views it moves the guns.
The default sensitivity (slider in the middle is the default? I don't think I ever moved it, but of course it can be a default inherited from ages ago through updates, and not be todays default) seems pretty coarse. The smallest physically possible movement with the mouse causes a signfiicant step (to provide an example of how coarse: bomber gun, looking at a drone, one tick moving the mouse from left off an engine to the right of the engine - but impossible to aim at the engine itself). Maybe the default should be much further to the left?
Note: I had "Disable all mouse controls" set since about ever. Something never felt right, and I never bothered to re-check. It looks like it may have been that invert not working thing.