This has been discussed extensively among the German-speaking SBB Community (https://community.sbb.ch/t5/SBB-Mobile-SBB-Preview-EasyRide/Swiss-Pass-auf-Apple-Watch-Wallet/m-p/66999#M8376))
There are no plans to support watches for SwissPass / SwissPass Mobile. For security reasons, this is not yet possible.
Also, please keep in mind that, no matter what device you use to show the SwissPass Mobile, you are required to have the SwissPass card or an official ID document always with you when using the Swiss Pass Mobile. It is not valid otherwise, more information here: https://www.swisspass.ch/enswisspassmobile