I don't think Hikki has undergone plastic surgery. She isn't really enough of a "public figure" per se for that to be a practical marketing choice, so unless she had a personal reason- sometimes people DO get nose jobs or certain touch-ups because they have preexisting conditions. My best friend got plastic surgery on her face because she had a bone growing the wrong way or something like that; it wasn't life-threatening, but in the end it was more convenient for her to do the deed and be done.

And really, plastic surgery has become mistakenly identified with vanity and superficiality. There are many tangible benefits to it, just like with any other form of surgery which is perhaps not necessary but may enhance a person's standard of living in some sense or another. I trust Utada as an intelligent human being to do what she believes is best for her own body.