It depends on the nature of the fanfiction. If it adheres to the general rules of respect and propriety, and doesn't involve inappropriate or offensive content, it can be an acceptable form of creative expression within the fan community.
In general, it can be appropriate. Fanfiction is a way for fans to expand on the stories and characters they love. If the 'Camilla x Corrin' fanfiction is written with respect for the source material, with a focus on character development, and is shared within an appropriate community that has similar interests and ethical standards, then it's a valid form of creative writing.
Well, it can be appropriate as long as it stays within the boundaries of good taste. For example, if it focuses on building a positive relationship between the characters, exploring their personalities in a new context, and not delving into anything vulgar or against the spirit of the original work. However, one should also be aware of the potential sensitivities of other fans.