No, asking about popularity shouldn't be on topic.
Popularity is hard to define. Something may be liked by the majority of people among one group of people but not among others. As such, if a question asks about the popularity of something, the population in which it is/isn't popular in question must be specified explicitly.
Generally speaking (I can't even think of a reason where this isn't the case but it's possible I suppose), there is not one or two reasons why something becomes or un-becomes popular. This leads to opinionated and list type answers, which the SE format is not well suited for.
Correspondingly, the usual answers for these types of questions would be very similar (unless there is one or two specific reasons why it became/un-became popular), including:
- It fit the culture at the time
- It matches a structure that people are familiar with
- People associated it with something else that they liked
- It was a progression from the period before it
I'm sure there are others, but this type of principle could be applied to most all questions regarding popularity. All someone would have to do is replace "X" for "Y" in the question and it'd still be valid but have answers that are essentially the same. Duplicating the same answer with only a few things changed doesn't help anyone because the answer from one is valid on the other.
This might lead someone to believe that having one or two more broad questions/answers, like "Why does music become popular?" and the opposite, "Why does music become unpopular?" would be good, but then we face the problem that a question like this is overly broad. While I'm more okay with the thought of something like this, there is no specific set of reasons that can be applied to every song/genre/artists/etc. Each of these principles may be applied in specific ways for each song/artist/genere/etc.
With that being said, more specific questions can be on topic through the application of the principles of why something became popular/unpopular. For example, I could see something like the following being on topic:
How did [this event / something else] make [X] no longer popular?
This question can be answered by saying, "it didn't" or "It did in X, Y, Z ways" along with detailed explanation of that answer.
This specifies the population, as mentioned above, and asks a clear and non-overly broad question.