Likely answer

Graphic Novels About Being Trapped in a Fantasy World

The book you are looking for is "I Hate Fairyland, Vol. 1" by Skottie Young. The series follows Gertrude, a girl who has been stuck in a whimsical, colorful world for thirty years. Because she hasn't aged physically, she remains a small child on the outside but has grown into a violent, cynical adult on the inside, desperate to hack her way out of the cute world she despises.

Confirmed match: I Hate Fairyland, Vol. 1 by Skottie Young