Ayane: Black leather has been her thing since, what DOA2 or 3? Fits her fine.
Kasumi. She wears bright colors and soft outfits. Black leather should be nowhere near her. Its why her black leather outfit in DOA5 looks so dumb.
Actually Ayane has never got a black leather, if something dark outfits (like her DoA3 C1): however the black leather is intended to represent her "serious" shinobi personality and this is exactly the reason because TN decided to modernize Kasumi's vestiary with a similar black one. In the game is well represented the difference between her soft clone (that wear her old standard blue outfit and has the old Kasumi's personality) with the real Kasumi that now appear more determined than never before (and that wear a more serious and dark outfit); even her quotes appear more determined in fighting the opponent and this was a process already started with DoA4.
The appearence of Kasumi in the Ninja Gaiden series (united with her "new" personality) has the purpose to give the message to the players that this is a more serious and deadly Kasumi, but of course this doesn't mean that she lost her nice side: like already done by other fighing series, the characters are not static but change their personalities with the time, not for "mistake" but for a precise choice of the developers (and DoA5 it's a perfect example of it).
Everyone has their own preferences, so it's possible to like or dislike the changes. But IMO is wrong to say in absolute that a character should/shouldn't ever to appear in a way rather than another, overall when the reasons for doing it appear logical. While TN could have exaggerated in giving to Zack a joke aspect (on the opposite for example with Bass, where according to me they balanced better his serious old personality with a new more sympathetic and light one), nowadays the model of the heroine is a woman with good intentions and skilled, but that balance her femininity with a strong side that has nothing to envy to the male counterparts.
Even if far from being a "damsell in distress", the old Kasumi would have appeared "weak" and too much soft given the current canons (it's enough to think to the clone like the main character of the game for realizing it), so no surprise by me for this choice of TN that in my opinion was really smart for renewing the popularity of what is very likely their most famous character.