I've never used "mods" in multiplayer. Save editing, however, I still do; using in-game assets. For example, before the tuning pack applied to the new Scanias, I took the lightboard from the old Scania and put it in the back of the new Scania. I also use the Volvo D16 750 HP in a few different trucks, and in my Volvo, I've changed my grill from that horrible chrome to the black plastic grill, or used the visors from the DAF on my Volvo.
I don't see why you'd want to use mods that no one else would be able to see anyway. I mean, half of this game is simulating driving a truck. The other half is looking good while you're doing it.