She should have performed a song he actually made popular period.
In general I've been surprised at how many artists have chosen "NC2U" as tribute, but then maybe it has to do with its message (saying no one compares to Prince) and even perhaps recognizing he didn't like cover versions of his songs (covering a song that wasn't directly associated with him versus a song that was truly his).
Meanwhile, I listened to "Love Song" today (i.e. the song Madonna wrote and sang with Prince). Madonna sounded pretty bad on it, no? That's my issue with her. Some of her songs are probably great, but so many of them are ruined by her singing to where I can't get into them.