I'm not sure about its popularity precisely. While the Harry Potter fandom is huge, this particular combination of Harry and Basilisk in a 'lemon' context might be more of a niche interest. Some people might be put off by the idea as it goes against the more traditional interpretations of the story. But on the other hand, there are always those who like to explore strange and unexpected pairings in fanfiction, so it could have a small but dedicated following.
Popularity can be a relative term. In the broad scope of Harry Potter fanfiction, Harry and Basilisk lemon fanfiction is likely not very popular. The mainstream Harry Potter fanfiction usually focuses on more conventional relationships like Harry and Hermione or Harry and Ginny. However, within certain sub - groups of the fandom that are interested in more off - beat or adult - themed fanfiction, it might have some level of popularity. But overall, it's not something that would be widely known or accepted by the general Harry Potter fan base.