I actually commend Nikki for trying to be dedicated to the role. Her hair fell out because of it and I was relieved once they wigged her in the other films. Hardwicke had a hard time finding a Rosalie and she went with Nikki because of her acting, which I prefer a good actor over a face any day. Plus Nikki Reed is gorgeous.
She takes a lot of unnecessary hate from the fandom for speculation and hearsay about her, but her actions, words, and charity work usually go unnoticed. She is in one of those situations that no matter what she does, someone will find a way to twist it into a hateful thing. Anything she says will be somehow used against her. Out of all the original cast, if I were her I would probably be the one who is ready to move on the most and get away from the haters for a bit, but who knows. She seems to really love her role and plenty of the cast seem to have mixed feelings like she might have. They want to move on and do something else, but also know there will never be anything quite like Twilight again. That is just my opinion though based on interviews and press.
I think her hair looks fine in NM and Eclipse, so while I agree about her hair in Twilight, it isn't a big issue for me anymore. I liked the Jasper hair in Twilight and was so confused with the mop they used in New Moon. It was better in comparison in Eclipse, but I haven't seen enough of the BD hair to decide yet. The Eclipse hair was either hit or miss though for most fans. Some loved it and some hated it, even when SM backed it. I know he gets to use his real hair in BD so at least it won't look fake. They even changed Edward's hair too much for me, sideburns, no sideburns, cut shorter or shaggier, it's like make up your mind and even Rob comments about it a lot. Kristen's hair issues were her choice so I wasn't annoyed with Summit on that one. She decided that chopping it all off to play Joan Jett instead of wigging a mullet would be a good sacrifice. In turn, the hair looked horrible in Eclipse, but at least it will be back to normal in BD. Peter's hair seems to be changing a bit as well and his isn't as noticeable, but it is different. I loved the pixie look in Twilight, but understand why Alice's hair has changed. I don't like it, but I understand it.
My biggest issue has to be the makeup. I actually like the complexion in Twilight, but in New Moon and Eclipse it was extremely obvious that it was caked on makeup. I loved the contacts in Twilight because they didn't look overly obvious which I think was supposed to be the point. In New Moon and Eclipse, they look yellow and stand out so much, but in Twilight the eyes were a great golden brown that didn't look weird. The lack of consistency has always been a punchline with these films. I am not sure who to blame with that, but Jasper's hair is by far the biggest change.