Armyvet: the fact that non-clerics wear the clerical garb when in clerical function or as seminarians is abundantly approved by the Church, for centuries. It’s not just a practice of one period, but quite an old one that is not just allowed but in many places has been mandated as seen as essential.
Girls do not fit into this. They have been allowed to serve at the altar, but the Church hasn’t clarified the practical matter of how this is to be done in terms of dress. There is no tradition on this to guide us either.
However given what the cassock means (clergy, seminarian, or server acting as clergy in a way that makes sense - a male), a girl server doesn’t fit into it.
The Church has also always taught that the dignity of women doesn’t depend at all on their being seen as similar to men. The idea that women need to dress similarly to men is from secular feminism, not from Catholicism.
This is the trad catholic forum. There are people here who understand that just become Rome allows a practice doesn’t mean it’s the preferred practice, and it definitely doesn’t change reality. The reality is that looking at the entire history of the Church, girls serving is an anomaly, they don’t fit in the same way boys do, we do have an all-male priesthood, we are working towards reunion with the Orthodox who would consider this just another silly Catholic innovation and who don’t have women in their sanctuaries except in rare circumstances.
When a girl serves, it’s different than when a boy serves, it’s okay to distinguish this with attire. It’s not different functionally necessarily, but ontologically.