Why everyone hatin on the change?
Well, I can only speak for myself on this so to put short and simple; I felt the change came out of nowhere and did not seem to fit Ashley William's characterization especially since she always seemed to keep a very practical look according to my memory.
I wasn't because it seemed odd given how Ashley had always worn her hair in a simple bun and always worn the military uniform off-duty yet now she wears her hair long and wears what is not the Alliance's military uniform or a version of it. Ashley always seemed to prefer a practical and simple look so I am not sure where the look she had in Mass Effect 3 came from or why they decided to give her it.
"Ashley, Kaidan and Liara were meant to be love interest throughout all three parts of the triology. After taking them away from player in Mass Effect 2, they were ready for a passionate return in Mass Effect 3. For Ashley's reappearance in the series, we let her hair down and gave her sex appeal". - From 'The Art of Mass Effect'
Basically they wanted to make her more sexually appealing. Honestly I'd rather they stay true to the character's instead.
It seemed like the developers changed Ashley William's appearance not as a result of her characterization and not as a result of character development but because they wanted her to be eyecandy for the player.
That kind of thing has seldom, if ever, sat well with me.