I initially considered this as just an irrelevant part of the post, analogous to users telling some completely unrelated story. Since those should be edited out, I believe that unnecessary disparaging remarks should also be removed. In the case User topo morto specifically mentioned, the post is fine without the remark.
Most of the time, the post just makes some passing comment, and the opinionated part can just be replaced or removed, and the answer is still a good answer.
I'm not aware of any specific ruling on this, but I imagine it is covered by "claims should be backed up by evidence". (Interestingly, that means the "formless" part would be okay if they had evidence that the person was writing music that didn't have any discernable form)
I think that the important factors are subjectivity versus objectivity and relevancy. If the poster can point to something that they don't like about the music, such as "this artist only uses 4 chords", then I probably wouldn't remove criticism as long as it wasn't profane or too irrelevant. If someone asked about parallel fifths in a Beethoven piece and then someone answered with "Beethoven just wrote formless drivel and called it music", that would be unnecessary.
TL;DR: Generally, that's not okay.