No, it's not appropriate. Pokémon is a franchise aimed at a wide range of audiences including children. This kind of fanfiction involves adult and inappropriate themes that are completely out of sync with the family - friendly and positive image of the Pokémon world. It can also be seen as disrespectful to the original intention of the characters and the story.
The Pokémon series has always emphasized values like friendship, courage, and the bond between trainers and Pokémon. Introducing such a concept as pregnancy in a sexual context is not only inappropriate but also goes against what the series stands for. It can make the Pokémon community seem less welcoming and family - oriented.
There are so many positive and creative aspects of the Pokémon world that could be explored in fanfiction. Writing about things like Ash and Serena's journey, their Pokémon battles, or their dreams and aspirations would be much more in line with the essence of Pokémon. So, this type of inappropriate fanfiction should be avoided.