As a reviewer, is it inappropriate to make unsolicited comments on another reviewer's comments? I'm reviewing a paper after minor revisions, and can partially see another reviewer's comments in the response letter.

The other reviewer asks the authors to do X, but the authors argue that it's not worthwhile doing X.  I've done things like X myself, and have found it not worthwhile (for the precise reasons the authors state).  Thus, I strongly side with the authors on this matter, and I'm thinking about whether I should inform the editor.  I'm not sure if I'm meant to do this...

Question: As a reviewer, is it inappropriate to make unsolicited comments on another reviewer's comments?

A related question is In peer-review, is it common for a reviewer to be shown and asked to comment on other reviewers&#39; reports?  However, I have not been asked to comment on the other reviewer's comments.
It is appropriate. You're acting in good faith after all, the editor is likely to be interested in what you say, and you're not harming anyone.

This is however the kind of thing to put in the confidential comments to editor box. The authors don't need to know of friction between reviewers.