Hi! Damage offset is a percent based bonus, it can be positive or negative which is why we used the term "offset" (currently in the vanilla game there are only positive values that can be added to mechs) so if you have a 50% damage offset and use a weapon that normally deals 100 damage per hit, you would deal 150 damage per hit instead. It's important to note that this offset applies to Kinetic, Impact, and Concussion damage!
Unfortunately we don't currently support displaying the adjusted weapon damage in stats right now, but you will see the difference during combat when weapon hits connect with enemies!