I've always felt one should dress to one's body type, but not to one's hair or skin color. This applies to dudes cosplaying even more than women, though, because I do not need to see any more Boba Fatts or Stick Figure Akumas.
That's not to say you gotta be PERFECT, just... if you are overweight, it might not be the best idea to do a Soul Cal Cosplay. Especially not Voldo.