People who write fanfic are doing the best they can.
Sometimes that best is pretty rough. You can’t know what constraints they’re working with. Education, language barriers, learning disabilities, technology issues, illness (physical and mental), abusive home lives… there are so many, many things people struggle with who post free stories on the web.
Yet they still put themselves out there and share their work.
When I see people complaining about update times or mocking spelling issues or bitching something isn’t British enough it makes me sad. The writer did the best she could under circumstances you do not know, and no one made you read it. You chose to read it.
You could choose to be kind too. You could choose to uplift instead of tear down. You get to make that choice.