Being as objective as I can, I'd say that the way female characters as a whole have been treated has gone down as the series has progressed. I kind of see it as Pre-Skypiea/Post-Skypiea:
*Pre-Skypiea, you had several tough-as-naiils females either introduced or expanded upon. Most of the characters widely regarded as codifiers of badass females in One Piece were introduced here, such as Tashigi and Hina. Additionally, Robin and Nami's chickification was not yet totally established at that point, and even the girliest character (Vivi) had several great moments, a good tomboy streak, and a lot of responsibility.
*Post-Skypiea, you begin to see the treatment of females go down. You have a constant dichotomy of ugly versus hot adult females, the relegation of Nami and Robin to non-action roles (particularly jarring in Robin's case, since she actually made it to the end of the survival games), a character with soap powers, a whole ISLAND of scantily clad amazons wanting Luffy's kintama, a hot chick with the ability to turn things to stone with HEARTS, an old woman with washing powers, and a much lower ratio of action girls to action guys.
I'd be interested to see what a counter opinion would be on this, but the way I see it, things got much worse for female characters. That doesn't make me love One Piece any less, mind you - I just don't see it as a shining example of equality of the sexes.
Now, gender, on the other hand…That is something that Oda's done a really damn-good job of handling, lol.