There are two ways of categorizing music: by how they were regarded when they were originally written/performed, and how they are currently regarded. Minstrel songs from the 15th century were that generation's pop music, but now they are regarded as artsy. The divisions will change with the social groups asked (see my post about the Social Psychology of Music) and the labels used for the divisions. One could divide music by the intent of the composer: to make an artistic work, to make money, or to become famous. This will definitely be blurry, and difficult to easily discern. One could divide music by the venues it is performed in: stadium/arena, concert hall, night club, dance hall, etc. This gets more to how we regard the music, by where we as a society let it be played. How about the education of the composer, or the instrumentation (electric = lowbrow)?
My preference is to describe any piece on its own merits, comparing it to other pieces as appropriate if they share similarities in compositional technique, lyric subject, social function, etc. I've been finding that I enjoy more and more music with this approach.
What I really want to say is that any attempts to judge the aesthetic worth of a whole genre of music will fail, unless the music of that genre is so homogenous that shared characteristics are not of a trivial nature. What characteristics do all popular musics share, that "artistic" musics do not? Nothing comes to mind. So let's focus our attention on judging the worthlessness of individual pieces or musicians. Brittany Spears sucks!