This claim is true.
The TikTok user who posted this claim, Allyn Aston, is a self-titled "pop culture enthusiast" who makes videos about breaking celebrity headlines and often includes her own opinion. The reasoning she gives as to why she believes the image of Kate Middleton is photoshopped is because The Associated Press issued a kill report.
In this article, the global news organization explains that it removed the image from their site because it was manipulated. In a statement to ABC News, The Associated Press said, "At closer inspection, it appears that the source had manipulated the image in a way that did not meet AP's photo standards. The photo shows an inconsistency in the alignment of Princess Charlotte's left hand."
The ABC News articles reveals that according to the metadata on the file, the image was processed through Adobe Photoshop twice.
My research shows that this claim is true. The image was photoshopped. However, the original TikTok video claims the image was taken from a separate event that Middleton and her children attended in November. Aston compares the similar clothing the family wore at the event with the clothing in the photoshopped image. I didn't find reliable news sources discussing the origin of the image, so I can't conclude if Aston is correct.