Mediatrix is a proper title for Mary, but we should keep in mind that making Latin nouns masculine (e.g. medaitor —> mediatrix) often provides subtle changes to meaning. Another proper title for Mary is Genetrix (Ora pro nobis, Sancta Dei Genetrix, from both the Angelus and the Rosary). Genetrix comes from the same Latin verb, generare, meaning to beget or bring to life. We see this same verb take on a different noun form in the Latin Creed:
...Et in unum Dominum, Iesum Christum, filium Dei unigenitum...
In this case the unigenitum is Jesus, literally, the only begotten Son. Just as significantly, the genitor, or begetter, is God the Father. And so, genitor (masculine) means begetter or father while genetrix (feminine) has an equivalent meaning in reference to a woman: the one who brings forth, which is to say, mother. These subtleties of semantic expression are often lost in English texts because English rarely uses word inflections (e.g. genitor for father and genetrix for mother) to distinguish such shades of meaning. This is one reason why it can be perilous to draw some conclusions from an English translation of Scripture — unless one is aware of the linguistic and cultural norms of the the society in which the Scripture was written (the phrase, James, brother of Jesus, where brother means cousin or other relative, comes directly to mind here).
But back to Mediatrix, a proper title for the Blessed Mother. Mediatrix is certainly the feminine form of Mediator, but Mediatrix does not merely take on the same meaning as Mediator when applied to a woman. Rather, it takes on a specific meaning that is proper for the Mother but distinct from the meaning of the masculine term Mediator, properly applied to the Son. Frankly, my Latin fails me in understanding this subtle difference, but no matter. We can simply turn to John Paul, writing in Redemptoris Mater:
At Cana in Galilee there is shown only one concrete aspect of human need, apparently a small one and of little importance (”They have no wine”). But it has a symbolic value: this coming to the aid of human needs means, at the same time, bringing those needs within the radius of Christ’s messianic mission and salvific power. Thus there is a mediation: Mary places herself between her Son and mankind in the reality of their wants, needs and sufferings. She puts herself “in the middle,” that is to say she acts as a mediatrix not as an outsider, but in her position as mother. She knows that as such she can point out to her son the needs of mankind, and in fact, she “has the right” to do so. Her mediation is thus in the nature of intercession: Mary “intercedes” for mankind. And that is not all. As a mother she also wishes the messianic power of her Son to be manifested, that salvific power of his which is meant to help man in his misfortunes, to free him from the evil which in various forms and degrees weighs heavily upon his life (Number 21, emphasis mine).
“She acts as a mediatrix not as an outsider, but in her position as mother.” Herein lies the difference. Mary is not the Son but she is the Mother — our Mother as well as His. And this state of being (i.e. universal Motherhood) is what defines her role as Mediatrix.
Masculine and feminine inflections in language are not merely syntactic devices, put in place to avoid embarrassing each other in speech or writing. Rather, these syntactic devices reflect deeper Truths about who we are as persons and as aggregations of persons - that is to say as peoples or cultures. These meanings can only be understood properly if we do not obscure them, either by explaining away masculine and feminine linguistic distinctions on a superficial level or by hiding the explicit masculinity and femininity in our sacred texts by replacing it with neuter language.
If we are to change the text, I would change “We always find in Mary the perfect mediator of God’s grace to mankind” to “We always find in Mary the perfect mediatrix of God’s grace to mankind,” in order to emphasize Mary’s proper role.
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