Toss the zucchini with 1/2 teaspoon salt in a medium bowl; let stand 10 minutes. Wrap the zucchini in a kitchen towel and squeeze dry.
Meanwhile, heat the butter in a large nonstick skillet over medium-high heat. Add the onion and garlic and cook, stirring occasionally, until slightly softened, about 4 minutes. Add the corn and cook, stirring occasionally, until crisp-tender, about 3 minutes. Set aside.
Whisk the cornmeal, flour, baking soda, 3/4 teaspoon salt and 1/4 teaspoon pepper in a medium bowl. Whisk the buttermilk and egg in a large bowl, then stir in the corn-onion mixture and zucchini. Add the cornmeal mixture and stir until just combined.
Wipe out the skillet the corn-onion mixture cooked in. Pour in about 1/8 inch vegetable oil and place over medium heat. Working in batches, scoop scant 1/4 cupfuls of the batter into the oil and use the back of the measuring cup to flatten the scoops. Cook until the fritters are golden brown, 3 to 4 minutes per side. Drain on paper towels and sprinkle with salt. Serve warm or at room temperature. (You can make the fritters up to 2 hours ahead; reheat in a 375 degrees F oven on a rack set on a baking sheet.)