MILD SPOILERS FOR BOTH GRAPHIC NOVEL AND FILM
I thought this movie was pretty great, but I feel that it should be considered as INSPIRED BY the graphic novel rather than an adaptation of it.
There were many notable differences in character development, timelines, lore, and general plot to the point that I don't think they're the same story whatsoever.
For those who have read the novel, the movie has no mention of a joust, jaderoot, Blackheart's devotion to science, etc. While the novel does show some tech, the movie is much more technologically oriented. B plot characters like Dr. Blitzmeyer are written out of the story entirely.
The main similarities are recreations of small scenes written to fit the plot of the movie, like when Nimona flipped the table because (in the book) she couldn't shapeshift due to Blitzmeyer's device or (in the movie) she scared a little girl when she shifted. There was also the boardgame scene, the zombie movie scene, and the meeting between Goldenloin and Blackheart at the Antlered Serpent. But again, all of these scenes have differing contexts in the book vs the movie.
One thing that I did like from the movie as opposed to the book was that it was good to see the gay lovers to nemeses arc in full cannon as opposed to STRONG implications in the book.
All in all, both are good stories, they just aren't the same story.