Because she loves the HTTYD world. Fanfiction can offer new and unique takes on the story she's a part of.
The main reason could be the shared elements of dragons. Vikings in mythology had interactions with dragons, and HTTYD is all about dragons. Fanfiction often explores different aspects of the dragon - human relationship, which could be appealing to those interested in Viking lore.
They might be interested because it gives them a different view of themselves. Fanfiction often takes existing characters and puts them in new situations or relationships. Asgardians could find it interesting to see how mortals perceive their powers, personalities, and relationships.
One reason could be that the official story has some aspects that Akatsuki wants to see expanded upon. Fanfiction allows for this expansion, whether it's exploring relationships between characters more deeply or creating new adventures for them. Another factor could be that Akatsuki enjoys the creativity of the fan community. Fanfiction writers often come up with very imaginative ideas that can be exciting to read or watch.
It's hard to say if it's common. Since Valka is a fictional character, we don't really have data on her fictional reading habits. But in the context of the fandom, it could be seen as something that would be cool if she did, as it would show her interest in the fan - created content.
Maybe they're just bored. After all their battles and saving the world, they need some entertainment. Batman fanfiction could be a new and different form of it.
Percy has had a lot of experiences that are both amazing and terrifying. Fanfiction allows him to see how others would have liked things to go differently or how they would have added their own twists. It's also a way for him to kind of relive his adventures from an outside perspective, which can be both entertaining and thought - provoking.
They might be interested because it gives them a different perspective on their own world. Fanfiction often twists and turns the original story, and Akatsuki could be curious to see how others imagine their actions and motives.
Ninjago might be interested in Ninjago fanfiction because it could offer new ideas for their own battles and challenges. The fanfiction world is full of creative minds, and there could be some brilliant strategies or concepts in those stories that they could adapt. Also, it could be a form of entertainment for them, just like how we enjoy fictional stories about other things.
Maybe they are curious about how fans see them. It's like looking in a mirror that shows different versions of themselves.
The Avengers could also be curious to see how fans view their relationships. In fanfiction, the relationships between the characters can be explored in much more detail, whether it's romantic relationships or just deeper friendships. It gives them a chance to see how they are perceived from an outside perspective. Also, fanfiction might offer them ideas for new strategies or ways to handle situations, as some fanfictions can be very creative in coming up with solutions to problems they face in the MCU.