yes, a quilt can certainly be 'over-quilted' if the quilting over powers the quilt-
quilting should enhance- not take over- and sometimes people seem to just get a bit 'carried away' and they fill a quilt up with so much quilting that it winds up being stiff as a board- and the quilt itself is lost somewhere under all that thread.
but it is a personal choice too- some people like very dense quilting- others like a large--easy quilting style-
but no matter what kind of quilting a person likes it should enhance the quilt