Could this really be considered a Mio cosplay? I mean, isn't this just a Fusou Navy uniform? I'm pretty sure Takei and Wakamoto have this uniform too. Also I always assumed that most Fusou witches carried katanas.

Perrine is not a member of the Fuso Navy. Perrine is also infatuated with Mio.

Seems like a pretty clear-cut cosplay to me.

The presence of the sword pretty much affirms it being a Mio cosplay.