I'm not sure "secondary character" quite qualifies them though... because this is about characters who exist in the show relatively outside the action of the active plot and business of the lead characters, who can comment on it with a perspective not accessible to the leads characters.
I almost submitted "Lifeboat" from Heathers... but there's no reason why one of the three Heathers, a main supporting role participating in the plot, would be a surprising choice for a song, and while Lifeboat comments on the goings-on of the leads, it is very much about her own experience too. In this sense I'm not even sure Playing Nancy qualifies. Ha.
Mamma Look Sharp would be interesting to submit, because it doesn't comment directly on the action or characters but the story it tells does on a higher life... which I think is very true of The Miller's Son too. I'm not sure Sondheim ever fully utilized this again in any other show.