Quite simply asked, novelizations are literature, but they started as "screen" works. Should they be considered on-topic?
They may be related to non-book works, but they are works of literature nonetheless.
They are written by writers and are textual and tell a story.
Moreover, they are frequently different from the movies, and often are more in-depth, covering internal thoughts and motivations in great detail.
(and sometimes, are better than the movie in some ways - my personal subjective example is Revenge of the Sith novelization, in which Anakin's character arc makes about 100x more sense than in the film).
As you say, they're written works. It doesn't really matter where they started off; every idea came from somewhere, after all. Being written works makes them on-topic (in just the same way that cinematisations of novels are on-topic at our sister site Movies & TV). Usual caveats apply: if you want to ask a question that's actually about a film, better to ask it somewhere films are actually on-topic rather than posting it here in the guise of a question about a novelisation.
It should be noted that quality of a written work isn't a criterion for its on-topic-ness ...