Is there a way to hide the Timer from some participants?

The BlueSky Timer joins your meeting like an extra participant, and Zoom doesn't offer a way to hide certain participants from some of your meeting attendees. There are some ways to edit the Gallery view that everyone sees if you're the meeting host, which you can read about here.

If needed, you can always change the Timer's Display Method "No Display" and use the Timer just for yourself in the Control Center.