Trim off the ends of the zucchini and squash. With a vegetable peeler, shave lengthwise in wide strips. Cut sweet corn kernels off the cob. Combine sweet corn, squash ribbons, avocado slices, cilantro, and red onion. Whisk together olive oil and lime juice, and then toss with salad until evenly coated. Crumble queso fresco over the top. Season with salt and pepper to taste.