Of course one performance may not hurt her ... but it does leave impressions.
Keep in mind that Utada fans, like ourselves will be supportive of her no matter what. But, others may not see it this way.

Most of the comments on itunes and youtube have been from Utada fans, or those who knew of Utada's music prior to "CBTM". In the weeks and months to come there will be definite opposition and harsh criticism (Look at BoA, she's already facing major criticism and it's only been a week since her debut)

All in all, Utada's management is going to need to do a lot more promotion. After this week, we'll see how much of an impression she'd make on the general crowd, but it's not easy to break into the U.S. Market.

(Lady Gaga's hit single spent an entire year unnoticed, Keri Hilson had to release 3 singles until she made a hit, etc etc etc etc )

I hope Utada spends more time in the States and realizes that she kind of has to stick around and push for her success here. Of course, as she's stated in interviews, I'm not sure if "major success" in the U.S. is really what she's looking for.