There is no doubt in my mind of course that "mail order" clergy generally speaking will not have the same type of training as other forms of clergy. I am also like you certainly opposed to Messianic Judaism for the most part and have concerns about some widespread teachings among Christians and parts of the New Testament that are unfair or have harmful consequences for the Jewish people. In fact the Episcopal Church is in the process of reviewing the lectionary to remove anti-Semitic content or associations. I still cringe during some of the lessons, particularly in the expanded three year cycle of readings.
But I am primarily concerned here with whether it is ethical to grant anyone who wishes it the title "Rabbi" simply for applying for a mail-order ordination. For example, there is a specific meaning of "priest" in Catholicism or Anglicanism, but it can also be a very broad term or have different meanings -- one might mean that one is a neo-Wiccan priest or priestess when choosing that title for example. Or one might simply use the term to refer to a sacrificial role, and that might fit into a broad emergent form of Christian ministry. Or one might wish to use this term to emphasize a belief in a universal priesthood. So even though the ordained priesthood means something specific to me within the Episcopal/Anglican tradition I come from I'm not offended that someone might use this title in a different sense in an interfaith organization, although such a person could not by virtue of that ordination claim apostolic succession as Anglicans and Catholics possess it. That would be false and thus offensive.
But the term "Rabbi" has Jewish associations. Is that term broad enough to be granted by interfaith organizations like this -- as I believe the words "reverend," "pastor," or even "priest" are or is this a case of misrepresentation and cultural misappropriation? In other words, hijacking an aspect of someone else's culture and passing it off as one's own?