How to add a comment using service api when uuid_service module is enabled
First, when creating a comment you must use POST. Here's an example:

POST: ?q=[my_endpoint]/comment.json

{
  "subject":"Testing...",
  "comment_body":{"und":[{"value":"Hello world..."}]},
  "nid":"123"
}


Secondly, from my understanding the uuid will automatically be generated server side, so you should not need to send it up to Drupal. And lastly, you use the PUT http method to update a comment, and in that case you can send along similar JSON, but include the cid value (e.g. 456) and exclude the nid value in the JSON object, and change the URL path to ?q=[my_endpoint]/comment/456.json

{
  "subject":"Updating...",
  "comment_body":{"und":[{"value":"Hello world again..."}]},
  "cid":"456"
}