I'm going to agree with Zantetsuken's observations. I believe to say that Yuna from FFX & FFX-2, Ashe, & Lightning look similar is stretching the truth. Ashe's face is based off a french person's while Yuna has a japanese(okinawan) face, & Lightning is a "female Cloud" so that would make her what, European - not french.

Yuna's first outfit is a kimono; I can see some similarity between Yuna's X-2 outfit and Lightning's but I think there are enough differences. Ashe's outfit does not look like Yuna's or Lightnings outfits.

There are only so many outfits/look a character can have before they get too eccentric and zany. Yes all three have a mainly white top, some color variation may have done good to better distinguish them. At the same time designers shouldn't aim for copying looks. I find that those three girls overall designs are unique enough and their personalities vary, Ashe and Lightning being the closest personality-wise.

Dodie brought up a good point as well: that older female & some male, FF characters look similar. It could be explained that the limiting technology in the characters' respective console could not offer variations, but Amano's designs do show similarity as well.