Spray potatoes with olive oil and season with salt and pepper. Place on baking sheet and cook for about 1 hour, or until tender and easily pierced with fork. Cool slightly. Don't turn off oven!
Cut potatoes in half; scoop out insides and transfer them to medium bowl. Mash insides with salsa, sour cream, cheese, salt and pepper (to taste). Put filling back into potato skins and return to oven. Bake for 10 minutes more at 350°F.
Heat olive oil in a medium saucepan over medium-high heat. Add onion and cook until softened, 2–3 minutes, stirring frequently. Stir in fajita seasoning and cook 30 seconds. Add zucchini and mushrooms and continue to cook, stirring frequently, 3–4 minutes or until softened. Add corn and cook until heated through, about 1 minute.
Top each potato with ¼ of the vegetable mixture and cilantro (if using).