I think my issue with the flower princess setup isn't necessarily the dress, it's how the whole thing doesn't quite gel together. Long dresses suck for combat, which wouldn't be an issue if we wanted to be a long-distance low-movement caster. But one of the weapons in the plan is the Rakuyo, which requires very high mobility to use. Good luck changing directions quickly in a dress that trails on the floor.
If you want to go for the ballgown, cool. Go ahead. But make it so the entire setup matches and makes sense. Maria's outfit and the Bowblade (or the Moonlight sword, for that matter) make sense for a versatile fighter hiding her eldritch traits. If you want to go in the other direction, then don't bother with a sword/dagger that she doesn't need and doesn't fit the rest of her design. Work towards using arcane blasts for distance work (laser flowers!), use tendrils for shorter range and battlefield control, and use buds for misdirection and deception. And if we need a melee weapon, well, I'm sure a plant like us is capable of making thorns.
TL;DR make a coherent character design, darn it. If you want to go for an eldritch plant-girl, go for an eldritch plant-girl.