No, it is not appropriate. 'Miraculous Ladybug' is a popular animated series aimed at a wide audience including children. Writing sexual fanfic about the characters goes against the family - friendly nature of the show and can be considered inappropriate and disrespectful to the original work and its fans.
Another reaction could be that Marinette pretends to cooperate with Adrien at first. By doing this, she can buy time and look for an opportunity to escape or get help. She might act as if she is okay with what is happening while secretly planning her next move, like leaving a clue for someone to find her or finding a way to disable Adrien temporarily.
She might befriend Iron Man first. Since he's into technology and Marinette is quite creative when it comes to making things. They could bond over creating new gadgets or improving existing ones.
Marinette can react by standing up for herself. She might confront Chloe directly and tell her that her actions are wrong. This shows Marinette's strong character.
It could be Bruce Wayne himself. Marinette might admire his business acumen and his philanthropy work in Gotham. His mysterious persona as Batman could also be very appealing to her, especially if she has a taste for adventure and mystery.
Marinette's character can be developed through her angry outbursts. For instance, when she gets angry, we might see her make rash decisions that she later regrets. This shows her impulsiveness when overcome by anger. Also, her anger could lead to her learning new ways to control her emotions, which is a part of her growth. Maybe she starts to practice meditation or some other calming techniques, which changes her character over time.
Marinette could start by gathering evidence of her own whereabouts at the time of the supposed crime. If she was at the bakery or with a friend, getting statements from those people would be a good first step.
One great fanfiction could be where Marinette discovers the Peacock Miraculous in a hidden attic in Paris. She has to quickly learn to use its powers to stop an akumatized villain that is causing chaos in the city. The story could explore how she combines the powers of the Ladybug Miraculous she already has with the new Peacock powers.
I really like the fanfiction where Marinette uses the Peacock Miraculous to create a sentimonster that can disguise as Hawk Moth. This allows her to infiltrate his lair and find out his true identity. It's a very exciting plot that keeps you on the edge of your seat.
One great 'Chloe and Marinette Fanfic' is 'A New Perspective'. It shows an interesting dynamic where Chloe starts to see Marinette in a different light after they are paired for a school project. It's really engaging as it explores their characters in a new way.