Flatbush, just because people are lax about tznius on Purim, doesnt really enter the fact of whether or not women should wear costumes.
If you think that everyone is looking around at other people’s costumes, but women attract no additional attention by wearing a costume, to me thats a man’s problem then, not a tznius issue. To be more clear – if ten people are walking down the street in bright orange shirts, and a passerby looks at the women more than the men, thats his issue. On Purim, the norm is different than the rest of the year, so she is just blending in.
If we got rid of every article that attracts attention, women would not be allowed to wear ANY jewelry.