We have a specific feature that allows you to reveal or hide all the comments at once.
Here are the steps for you to follow in order to that:
Step 1. Go to the board you want to reveal or hide the comments
Step 2. Click on the clockwork icon (settings)
Final Step. Now choose the option “Show/Hide comments” to see or hide all the comments from your board at once
Note: This option of “Show/Hide comments” is not persisted. In case you log out of the board and log back in, you’ll have to select this option again to see the comments.