Comments are Posted anonymously
The default for an unprovided uid property to a comment entity is 0, so with your current code it will always be owned by an anonymous user. If you want a specific user, you have to be specific in the request.

It can just go straight in the payload though:

{
  "nid": "401",
  "uid": "123",
  "subject": "Comment Subject sdfsdf ",
  "comment_body": {
    "und": [
      {
        "value": "Comments body sdfsnd fsd fds "
      }
    ]
  }
}


You have access to uid from the return value from the login, so it's easy to get hold of.