Line a rimmed baking sheet with aluminum foil, adjust an oven rack to upper-middle position, and preheat broiler to high.
Heat olive oil in a large nonstick skillet over medium-high heat. Add zucchini and cook (in batches if needed) until browned on one side, 4 to 5 minutes.
Transfer zucchini to baking sheet, browned sides up. Top each round with 1/2 tsp marinara sauce, a pepperoni slice, if using, and a piece of mozzarella. Broil until cheese is melted, 2 to 3 minutes.
Transfer to a platter. Sprinkle with salt and serve warm.