The truth or falsity of moral judgments, or their justification, is not absolute or universal, but is relative to the traditions, convictions, or practices of a group of persons.I found the definition useful, regardless of the validity of MMR. I now address its validity.
The "traditions, convictions, or practices of a group of persons" are not ends in themselves. Rather, they are (or were originally) aimed at the attainment of a "greater good" -- a moral imperative -- which is (or was) served by such traditions, convictions, or practices.
The moral impetus for those traditions, convictions, or practices becomes tenuous with the passage of time. As the generations roll by, the members of a group turn their focus from the original moral imperative to the traditions, convictions, or practices that once served it. That is to say, the group's morality becomes rote.
Because of this rote morality, the moral framework of the group becomes falsely identified with its particular traditions, convictions, or practices. (A good analogy can be found in the widespread practice of celebrating the Fourth of July without giving more than a moment's thought -- then or during the rest of the year -- to the struggle for independence or to the meaning of liberty.)
MMR is valid only to the extent that there is no moral imperative that cuts across groups of persons: nations, races, ethnicities, clans, tribes, religions, political parties, and the like. (I disregard -- for the moment -- exceptions to the rule, that is, sociopaths, who (a) are likely to be found in any group of more than a few members, (b) quite often force or connive their way into positions of power (it goes with sociopathy), and (c) surround themselves with sociopathic henchmen.)
The crucial issue, then, is the existence (or non-existence) of a universal moral imperative, one that is common to the people (if not to the leaders) of nations, races, ethnicities, clans, tribes, religions, and the like. Kant would say that there is such an imperative, his categorical imperative (in its first formulation): "Act only according to that maxim whereby you can at the same time will that it should become a universal law." 
Kant's categorical imperative, however, is a Platonic universal: something that just is, a deontological duty. Kant, himself, distinguishes it from The Golden Rule, which (because of its commonality to so many forms of religion and philosophy) can be understood as a man-made utilitarian or consequentialist command. The Zoroastorian version, for example goes: ""Whatever is disagreeable to yourself do not do unto others."
Why is The Golden Rule utilitarian or consequentialist? Because people have learned -- from experience over the eons -- that if most everyone follows the command to "do unto others as you would have them do unto you," most everyone will benefit from doing so. One's self-restraint with respect to others encourages (almost all) others to practice self-restraint toward one's self. The Golden Rule does not apply to rule-breakers, who must face consequences (or one kind or another) for their rule-breaking. (That there are rule-breakers only underscores the humanness of The Golden Rule.)
Now, to answer the question of the title: Metaethical Moral Relativism, as defined above, is neither neither a valid concept nor an invalid one; it is an irrelevant concept. It treats different groups as if they had different moral imperatives. By and large, they do not; most groups (or, more exactly, most of their members) have the same moral imperative: The Golden Rule.
There are, of course, groups that seldom if ever observe The Golden Rule. Such groups are ruled by force and fear, and they deny voice and exit to their members. The rulers of such groups are illegitimate because they systematically try to suppress observance of The Golden Rule, which is deep-seated in human nature. Other groups may therefore justly seek to oust and punish those despotic rulers.
There is a relevant -- but logically and factually invalid -- form of Metaethical Moral Relativism:
The United States is imperfect. It is, therefore, no better than its enemies.Such is the relativism we see in those who excuse despotic, murderous regimes and movements because "we asked for it" or "we are no better than they are" or "war is never the answer" or "one man's terrorist is another man's freedom fighter" or "terrorists deserve the protections of the Geneva Convention." That kind of relativism empowers the very despots and terrorists whose existence is an affront to The Golden Rule.