Issue #16321 open
With the New Source Browser beta feature enabled, users are not able to see the option to clone from Bitbucket Mirror. Disabling this beta feature is the current workaround to have that option.
- Navigate to Bitbucket Settings >> Labs
- Disable the feature "New Source Browser experience by clicking the slider
- Refresh the repository and click "Overview" tab and you should be able to see the option to select Bitbucket Mirror.