“its just not how a Bas Yisroel acts even on Purim.”
Then by all means, do NOT dress up on Purim. I personally do not do so. I see absolutely nothing wrong however, with other women, who want to dress in the wholesome spirit of the simchas Purim, doing something that makes them feel special and festive, besides preparing the shalcha manos as well as the seudah for the rest of their family, or watching the men, who are busy getting shikker, and acting foolishly…er… lightheartedly… have fun. I wish people would stop being such straight arrows ONE day of the year, and learn how to show their kids that v’nahapoch hu, does not only apply to men. If the outfit is inherently tzniusdik and not overly ostentatious, chill a little, and stop being so judgmental for 24 hours. And if you can’t, then as I said , please do not wear a costume yourself. But don’t spoil it for anyone else who might be so inclined, unless there is a halacha accepted by the klal that b’ferush and in these words states that it is absolutely assur for a woman to wear a costume on Purim. If so, then there is nothing further to say. If not, ditto.