As she began to sing, Celine picked up a stuffed animal to hide her face, mimicking Sia's habit of hiding behind her hair. Then, as she got into the song, ramped up the tempo and started reproducing Sia's squeaky vocals, Celine completely covered her face with her own hair, much to Jimmy Fallon and the audience's delight.
Celine Dion was playing a game meant in good fun on a late-night talk show. I don't believe there was any malice toward Sia or any disrespect in the impression Dion did of the singer. I thought it was done very well, along with the other impressions in the bit, and was funny. Sia should not take issue with being impersonated by Dion.
I think that Celine Dion's impression of Sia was funny, and it was also spot-on and impressive! Sia shouldn't be mad about this. Her whole career is built on being unconventional and straight up weird. If all publicity is good publicity, she should be flattered that Celine has chosen to do an impression of her.
Celine Dion proved that she is funny and has a great sense of humor when she participated in the Wheel of Impressions game. She has a great sense of humor and was able to pull off an impression that was both charming and funny, by capturing and exagertating Sia's personality.