Mean, ignoble, abject, sordid can all be applied to persons, their behavior, or the conditions in which they live with the meaning so low as to be out of keeping with human dignity or generally acceptable standards of human life or character.
Mean usually suggests such repellent antisocial characteristics as malevolence or cupidity. It almost invariably connotes small-mindedness.
Often mean implies conduct or an attitude that is detestable and unworthy of a human being.
Ignoble, like its opposite noble, usually implies qualities of mind or soul. It frequently comes close to mean except that it seldom connotes smallmindedness. Its distinguishing implication is loss or lack of some essential high quality (as spiritual elevation, moral dignity, or intellectual excellence).
Abject, in its most inclusive sense, means little more than extremely low in station or in degree.
Sometimes it is merely an intensive applied to something that is itself low in the scale.
In discriminative use, however, abject carries the implication of being cast down and so variously implies abasement, debasement, or contemptible servility.
Sordid emphasizes the degrading baseness associated with physical or mental corruption.