Definitely not appropriate. Fanfiction is supposed to be a positive way to explore the world and characters of a particular story. Such a description violates the respect that should be given to the characters and can also make many readers uncomfortable. It goes against the principles of good - quality fan - created content.
There are many great Hermione - centered fanfictions that focus on her intelligence and heroism. For example, some fanfictions explore her adventures during the Hogwarts years, like her solving complex magical mysteries. Others might depict her post - Hogwarts life, perhaps her contributions to the wizarding world in terms of reforming magical education.
If this is just a fictional story, it's important to remember it's not real. But if it makes you uncomfortable, you could stop reading or find a different story.
One possible reason could be that Hermione's behavior might have put herself or others in danger. For example, if she was being overly reckless in her pursuit of knowledge, like delving into dark magic without proper precautions. Sirius, having experienced a lot in the wizarding world, would be worried about her safety.
The story could also develop in a way that Hermione discovers new magical abilities or powers within herself that she didn't know she had because of this situation. Maybe she starts to develop a stronger connection to her mind magic, like telepathy or some form of advanced spell - casting that doesn't require the use of her legs.
She might initially be defensive. Hermione is proud of her intelligence and actions, so she could start by trying to justify what she did. For example, she could say that she was only trying to help or that she had good intentions.
She might develop by interacting with different magical creatures. For example, if she befriends a hippogriff, she learns about loyalty and respect for other beings. This could shape her character to be more empathetic. Additionally, her relationships with her teachers would play a role. A strict teacher might push her to be more disciplined, while a kind one could encourage her creativity, both aspects contributing to her overall character growth.
One popular story might be 'Forbidden Bonds' which explores a unique relationship between them. It delves deep into their characters and how they might interact in a different way than in the original series. The author creates a lot of tension and drama, making it an engaging read.
Often, it begins with Hermione trying to help Sirius deal with his post - Azkaban trauma. She uses her knowledge and compassion to reach out to him. As he heals, he starts to see her in a new light. Their relationship then grows as they start to understand each other better. The lemon parts usually are used to explore their more passionate and private moments, which are often written in a very tender and intense way, adding a new dimension to their relationship.
Most official platforms would not allow such inappropriate fanfiction. Fanfiction should be about creativity within the boundaries of respect for the source material and its characters. Instead of seeking such a fanfiction, you could search for ones that depict their relationship in a more platonic or mentor - mentee style, which are much more common and in line with the spirit of the 'Harry Potter' universe.