is_flagged with Views
You could Create a Custom Service Resource for this, but IMO it's easier to do this:


Enable the Views JSON module (which is a sub module of the Views datasource module). This will allow you to get JSON output from a View.
Build a JSON data document page display for a View
Add a contextual filter for a node id, this value will be grabbed from the URL path
Add a contextual filter for a user id, this value could be grabbed from the URL path, or default to the current logged in user, that's up to you
Add a relationship to the flag in the View
Add your desired field output and watch the results preview to see your JSON assembled
Add the necessary filter(s) to limit the results to a certain user/flag/etc.


You then can consume this "resource" by doing a GET on the page path to your View.