It seemed quite mild compared to the rest of the things that her opponents could do to her.
Keeping a trophy from a win was more of a trophy collector behaviour that could be found in psychopaths.
A trophy being collected was almost to symbolize power and control that would serve as a reminder of their actions and reinforce their sense of superiority.
There were feelings provided by collecting such things as seeking validation and satisfaction while also making it possible to relive their actions and maintain a distorted connection to the thrill of their actions.
Perhaps those traits were what defined the Tooth Fairy in a sense but it was also possible that all of it was just a part of her act.
It would be difficult to tell unless Ash got to know her more.
The decision of Ash had already been made from the moment that she secured the win but there was a slightly revolting feeling that existed in the pit of her stomach.