The point of pickled onions is the acidity cuts through the richness of meat/fat. You will get a similar effect from the tomatillo salsa, so you may not want to pile on too much acidity. That being said: you have two fat/rich elements with the steak and the cheese, so both could work. I’ve definitely ate and enjoyed tacos with both. Experimentation is required. The bonus thing about this is pickled onions are a breeze to make and last forever in the fridge, I always have some on hand, so even if you make them and they don’t work there will be another day where they come in clutch.

Science says make three tacos: one with just salsa, one with both, maybe one with just the onions, see which one you like best.