For Jira Serivce management, there should an ability to view nested comments 

Instead of having just a a linear comment tree, There should be an option to view nested comments.

Here replies to comments and replies to replies could be seen.

The option to allow threaded comments, linear comments or even comments at all could be on an issue by issue basis as well as having project / global defaults which you could fall back on (if unselected).

This is already implemented in Jira core. Same functionality would be useful for Jira Service management as well.