Flaked corn does lighten the body. Body is basically thick malty sweetness, so thinning out that malty sweetness with something that ferments completely lightens the body. (I'm not sure why Tobias is suggesting adding alcohol without sweetness doesn't lighten the body, diluting the sweetness with alcohol, or water, or anything non-sweet is the definition of lighting the body.)
Flaked corn does not actually completely ferment, close but not completely, like corn sugar does. Because it does not completely ferment it does add some corn flavor to the beer. It will lighten the body except in the case of beers already overloaded with sugars. The yeast will generally eat the flaked corn before barley because it has a higher sugar to starch ratio.