Washing-up liquid, or dish soap, should never be used to wash a car. Washing-up liquid is designed to remove food and grease from plates and can be too harsh on a car's paint. Instead, use a cleaning agent formulated specifically for cars.
Washing-up liquid doesn't contain the water softener that helps prevent water spots as the car dries. If a washing-up liquid advertises that it is good for skin, it typically contains emollients, which will leave residue on a car. The surfactants in washing-up liquid create a lot of foam that can be difficult to rinse from a car, and they are designed to remove food, not dirt, so they won't be effective at getting the car clean.