It's certainly true that the Mror dwarves have had a lot of trouble with Orc tribes. Whether that means the orcs are the standard evil raiding tribes or not is another matter. One thing to bear in mind is that the dwarves were originally sent to the surface in exile from the mountains' vast subterranean caverns, and whilst I don't think it's ever been specified, it's entirely possible that they emerged to find the orc tribes already in place, and promptly displaced them. It wouldn't surprise me at all if there's a lot of history between the orcs and dwarves in the Mror Holds, with plenty of mistakes made on both sides.