This is such a wild idea! Since Attack on Titan is a work of fiction, the idea of it 'watching' fanfic is purely imaginative. However, if it were possible, the Titans themselves might not understand the human - written fanfics at all. But the human characters like Armin could potentially learn new strategies or ideas from the fan - created stories. It could also cause some chaos as the fanfic might have different lore or interpretations of the world, which could either enhance or disrupt the 'canon' understanding of the Attack on Titan universe.
Well, it's a really strange concept as Attack on Titan is a fictional entity and can't actually 'watch' a fanfic. But if we were to be creative, maybe it would be like the characters in the series are observing an alternate version of their own story. They might be shocked, amused or even inspired by the different plotlines and characterizations in the fanfic.
If Attack on Titan could watch Attack on Titan fanfic, it could lead to some really interesting scenarios. For example, the characters might react differently to how they are portrayed in the fanfic. Eren could be either enraged or intrigued by a version of himself that is much more docile or much more aggressive than he actually is. Mikasa might be confused or pleased with how she is depicted in the fan - created stories. And the overall world of Attack on Titan could seem both familiar and strange when seen through the lens of fanfiction.