How to add a comment form in VBO to add the same comment to mutiple nodes?
Step 1

Use the Rules module to create a Rules Component, which includes whatever Rules Conditions you might need (if any), and with a Rules Action to create an entity, of type "Comment", whereas you specify some relevant parameters for that Rules Component, such as:


The node to which the comment is to be added.
The relevant options for the actual comment (such as the actual text of the comment).


Below is an example of a Rules Action that creates a comment (in Rules export format, use the Rules UI to import it in your own site):

{ "rules_add_a_comment_to_a_node" : {
    "LABEL" : "Add a comment to a node",
    "PLUGIN" : "rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "rules" ],
    "USES VARIABLES" : {
      "comment_node" : { "label" : "Node to add comment", "type" : "node" },
      "comment_body" : { "label" : "Comment body", "type" : "text_formatted" },
      "comment_subject" : { "label" : "Comment subject", "type" : "text" }
    },
    "IF" : [
      { "user_has_role" : {
          "account" : [ "site:current-user" ],
          "roles" : { "value" : { "2" : "2" } }
        }
      }
    ],
    "DO" : [
      { "entity_create" : {
          "USING" : {
            "type" : "comment",
            "param_subject" : [ "comment-subject" ],
            "param_node" : [ "comment-node" ],
            "param_author" : [ "site:current-user" ],
            "param_comment_body" : [ "comment-body" ]
          },
          "PROVIDE" : { "entity_created" : { "comment_created" : "Created comment" } }
        }
      }
    ]
  }
}


Step 2

Create a view of the nodes to which some comment is to be added. Then transform that view into a view that uses Views Bulk Operations, and use the technique documented in "How to use Rules to create a custom bulk operation for a VBO view?". In doing so, you'll then be able to use the Rules Component from Step 1.

PS: this answer is for D7 ...