Why is my potted zucchini wilting?

You may be overwatering...don't water unless the soil is starting to feel dry...don't just water to water. Try transplanting it into a larger pot and while doing that see what the root system looks like. You should see nice, healthy white roots. Be sure the current or new pot is draining properly too. One warning on growing zucchini in pots: They get HUGE!! In my garden, a single plant can easily be three to four feet wide! You will need a very large pot to support that big of a plant and so it doesn't dry out on you too quickly.