Salesforce Case Comment Notification Email Template is not available for customization.

If you are using Email-To-Case feature and in the Email-to-Case setup “Notify Case Owners on New Emails” is enabled, it will send Email Notifications to the Case Owners when the new email responses are received for the Cases.

In Salesforce Support Settings, if “Notify Case Owner of New Case Comments” is enabled, it sends Email Notifications to the Case Owners.

Kindly, vote for this idea:

https://ideas.salesforce.com/s/idea/a0B8W00000GdhrZUAR/new-case-comment-notification-email-template

To use custom Email Templates for the Case Comment Email Notifications, make use of Salesforce Flow or Apex Trigger. The standard options are not available for customization.