Window decals that are intended specifically for cars are not bad for a car. However, the way in which the decals are removed may cause damage to a car.
Applying the window decal does not cause damage to the car as long as it is done correctly, but removing it improperly could leave scratches or residue on the car's windows. Before removing a window decal, be sure to clean and soak the decal and surrounding areas. The use of vinegar or rubbing alcohol followed by the application of heat makes removal more efficient. A safety razor or dental floss are the easiest ways to remove a decal once soaked and heated. Once removed, use an adhesive cleaner to remove any adhesive residue from the window. Finally, rinse with soap and water.