I want all merge comments set as private. Can I make merge comments private as default?
No, there's no way to make merge comments private by default. If you want a merge comment to be private, you need to clear the Requester can see this comment checkbox before you complete the merge process.
If the ticket doesn't include any public comments, then this checkbox is automatically not selected and the merge comment is set to private.
For more information about merging tickets, see the article: Merging tickets.