Should I explain this is additional comments section of application?

Looking at my transcript, I thought colleges might wonder why my grade dropped from an A to a B in choir second semester.Is it worth explaining that I had several bouts of illness during the second semester of my junior year, which caused my grade in choir to drop from an A first semester to a B. The class was graded solely on attendance, and unfortunately I wasn’t able to make up any points.

Is this worth even explaining?

It is choir, not a core academic course. I would not. If it were more of an overall issue, I’d ask your GC to address in their recommendation.

I mean, it does count toward academic GPA, so I think that your counselor should briefly explain it if you don’t.

No. It looks pathetic to ‘explain’ a single grade drop from A to B in a non-core subject. If your illness had impacted all of your grades that semester, then a GC should note that. Otherwise, you just look grade-obsessed.