There does not appear to be a way to get all comments for a particular commit, just a way to pull up individual comments by id. However, if you don't know the id, you can't request the comment. See:

https://developer.atlassian.com/static/rest/stash/2.7.1/stash-rest.html

This feature could be used to work around the one-email-per-comment issue that discourages devs from commenting on pull requests (STASH-2839).