It depends. On one hand, ads can bring in revenue and potentially help the fanfic community grow. But on the other hand, if not done carefully, they can be annoying to readers. If the ads are relevant and not too intrusive, then it's a good idea. For example, if the fanfic is about a particular movie series and the ads are for related collectibles, it might be okay. But if there are a ton of random, unrelated ads popping up everywhere, it's not good.
Ads on fanfic can be a good idea in certain situations. If the fanfic has a large and engaged readership, ads can be a way to monetize that traffic. However, writers need to be cautious. They should consider the impact on the reader experience. Some readers might be put off by ads, especially if they are overly flashy or disrupt the flow of the story. So, it's all about finding the right balance between making money and keeping the readers happy.