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.
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.
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.
For many, it would be disturbing. Reading or hearing about such a situation can be difficult because it challenges the basic understanding of trust and commitment in a relationship. It might also make people think about the fragility of relationships and how easily they can be broken by such extreme actions.
One possible reason could be a curse gone wrong during a spell - casting experiment. Maybe she was trying a new and complex spell and something backfired, affecting her legs.
One alternative could be fanfiction that focuses on Snape mentoring Harry. For example, Snape could be teaching Harry advanced potions techniques or helping him understand the history of dark magic in a way that Dumbledore couldn't. This would show a more positive and educational relationship between them.
You can try fanfiction.net. It has a large collection of Harry Potter fanfics, and you might be able to find some great hermione spreads for snape fanfic there.
There could be a fanfic named 'Hermione's Understanding'. In this, Hermione tries to understand Snape better than anyone else at Hogwarts. She might break through his cold exterior by sharing her knowledge of magic in a new way. It could show a side of Snape that we rarely see in the original books, as Hermione's perspective is different from Harry's.