Popularity is an indicator of a quality (appeal). If the author intends to write something with wide appeal and succeeds, they're probably good at their job. Now something can be popular and read by many people without necessarily appealing to them, but that's another story.

What is important to keep in mind is that works of literature have more than one quality, and even "great" works exceed at often just a few, while being mediocre on other axis. Many are considered great merely for being first or having an outsized influence on works that came after, even though later works improved on it and did the same thing better!