I actually really like the tights to be honest

My head canon: her dad just bought a bunch of stuff at random because he's a gritty merc whose knowledge of fashion only extends to shoulder pads and fade haircuts, and then his awkward fully grown child who doesn't know anything that isn't about stabbing people had to make an outfit out of that somehow. People at the monastery think it looks ridiculous, but don't say anything because they assume she's making some kind of conscious fashion statement. And she of course has no idea, because she's Byleth.

Though this makes way too much sense

i also didn't like her design at first, exactly for the reason of "female version of a character wear more revealing clothing than the male one" that happens rather often, and that it was bit of an odd outfit for a mercenary/proffesor, but honestly I stop being bother by it and nowadays do like it.
it still doesn't change that it annoying when the difference between male and female versions of characters happen