New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  jsonString: "",
  
  actions:{
    
      reg () {
        
        //Your work
       	this.set('my', {
       						"name" : this.get('name'),
                 "mobile" : this.get('mobile'),
                 "email" : this.get('email'),
                 "password" : this.get('password'),
    		});
        
        //This is mine work
        var tempObject = {};
        tempObject.name = this.get('name');
        tempObject.mobile = this.get('mobile');
        tempObject.email = this.get('email');
        tempObject.password = this.get('password');
        
        this.set("jsonString", JSON.stringify(tempObject));
  
        //Note: Both are same but in your case we can't create an object dynamically.
        }
	}
});

templates.application.hbs

{{my}} {{jsonString}}
<br>
<br>
 <form {{action 'reg' on="submit"}}>
        <h1>Register Form</h1>
        <div class="form-group">
            <div class="label">Name</div>
            {{input class="input-form" value=name type="text"}}
        </div>
         <div class="form-group">
            <div class="label">Mobile</div>
            {{input class="input-form" value=mobile type="text"}}
        </div>
        
         <div class="form-group">
            <div class="label">Email</div>
            {{input class="input-form" value=email type="text"}}
        </div>
        
         <div class="form-group">
            <div class="label">Password</div>
            {{input class="input-form" value=password type="text"}}
        </div>
       
        <div class="form-group">
            <button type="submit" class="reg-button">Submit</button>
        </div>
    </form><br>
<br>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}