My singer doesn't have the diva trait, but still loses a very small amount of relationship during performance. Most of the time it's not really noticeable, but when she had reached level 10 my save bugged and the only way I could recover it was to copy the household to the library, delete the current house and then copy it back, which of course wiped out all relationships with the town. It was then obvious after the next performance that nearly all the spectators had taken a very minor relationship hit, as they were no longer hidden amongst the clutter of pre-existing relationships.
It was very small though, and not game breaking by any means. A quick perform for tips afterwards easily got rid of it. My singer doesn't have any of the new traits, as she was a pre-existing sim - perhaps some of the new traits remove the relationship loss.
Also highest monetary gain from a performance so far - 21600 after a star performance at the Ultimate venue. There's money to be made in this old singing game!