Whisk the olive oil, lemon juice, shallot and garlic in a large bowl. Season with salt and pepper. Add sugar, if desired (depending on the sweetness of your tomatoes). Add the tomatoes and gently toss. Marinate at room temperature, about 15 minutes.
Meanwhile, bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Drain in a colander and run under cold water to stop the cooking.
Add the pasta and mozzarella to the tomato mixture and toss. Stir in the basil and lemon zest, and season with salt and pepper. Refrigerate, tossing occasionally, until serving.