Aside from maybe a handful of projects that got lucky squatting on the right name (e.g. left-pad), are there any popular open source projects that have become popular on "that would be fun to implement" rather than "that's what is good for business" (understanding the user, answering issues, implementing requested features, etc.)? The chances of your fun overlapping the exact market need seems rather slim.

Even among unpopular open source projects, I expect most of them are published as a way to demonstrate ability to employers rather than "that would be fun". The latter projects do exist, but it is surprising if they make up most open source projects.