Mac OS - Is it possible to sign in to iCloud via touch id instead of using the password?
Not at this time.

Currently, TouchID allows you to


unlock your apple devices
Authorize payments/purchases from ApplePay, iTunes Store, App Store and the iBooks Store
Unlock/authorize 3rd party app usage (Developer supported)


About Touch ID advanced security technology


  You can use it to authorize purchases from the iTunes Store, App Store, and iBooks Store, as well as with Apple Pay. Developers can also allow you to use Touch ID to sign into their apps.


You may see this in a future update, but (IMO) it acts like a second factor in a 2FA authentication when trying to gain access into your iCloud account.