Considered a rabbi by who? That's like asking if Joseph Aloisius Ratzinger should be considered a pope.
No, it really isn't. Members of the Catholic church acknowledge as pope whoever is elected by the college of cardinals. In the present case, Joseph Ratzinger. Anyone not accepting him as the pope is, by the standards of the Catholic church, a heretic, and, in theory, is liable to excommunication from the Church. And that is well known to all.
But because of the movemental differences in Judaism that tend to confuse non-Jews, and the ways that most non-Jews don't really have any clear idea of what is and is not acceptable in Jewish thought, people who are not rabbis-- who, in fact, are not even Jewish-- are able to convince non-Jews that they have some sort of legitimate claim to the title of rabbi, or to the identity of being Jewish.
That's part of the reason that Jews for Jesus are so pernicious. They steal Jewish identity for themselves, cloaking their Christianity in it, and create false information amongst the Christian community that somehow it can be acceptable to be both Jewish and Christian at once. And even worse, they prey upon young and uneducated Jews, who may not fully comprehend what they are, and attempt to miseducate these naive youngsters to believe that apostasy is somehow Judaism. And because of the dearth of knowledge in the non-Jewish world about Judaism, and because of the education crisis amongst young Jews today, Jews for Jesus is often much more successful at their goals than would be a Catholic group who tried to deny the pope.
That just blew my mind!