I concur that it isn't actually a feeblemind causing dress, but I assume that this description was used because it is simply more descriptive than 'blue dress'. So the feeblemind causing part should be read as between "'s or something (why it isn't between quotes then, I don't know, but I find it hard to believe that anyone seriously thought it was a magic item that causes Feeblemindedness).
We do keep track of other characters miscellaneous clothing items (see belkars cloak or Haley's extra clothes), so the fact that the dress is listed shouldn't in itself be surprising.
Edit: Also:
Why is this listed under items when in the linked comic zz'dtri refers to it as "feats stuff"?