I highly doubt it's a popular fanfic given the rather inappropriate and unclear title. Most popular fanfics usually have more engaging and appropriate titles that give a better sense of the story.
Probably not. The title 'natsu boobs of the dragon fanfic' is not only a bit vulgar but also very unclear. Popular fanfics tend to have titles that are easy to understand and that draw readers in with an interesting premise. This title seems more likely to turn readers away rather than attract them, so it's not likely to be popular.
It's very unlikely that 'natsu boobs of the dragon fanfic' is a popular one. A good fanfic title should be able to intrigue readers and give them an idea of what the story is about. This title, with its inappropriate and rather confusing elements, doesn't seem to have the qualities that would make a fanfic popular. It may be something very niche or just a poorly named work that hasn't gained much traction.