App.jsx

import React from 'react'
import {mount} from 'react-mounter';
import HelloWorld2Container from './HelloWorld2Container'

mount(HelloWorld2Container, {someOther: 'props'});

HelloWorld2.jsx

import React from 'react' 

class HelloWorld2 extends React.Component{ 
  render() { 
    return ( 
      <h1>{this.props.title}</h1>
    ) 
  } 
}

export default HelloWorld2

HelloWorld2Container.jsx

import React from 'react' 
import {composeWithTracker} from 'react-komposer'
import HelloWorld2 from './HelloWorld2'

const compose = (props, onData) => {
  let userName = Metoer.user() ? Metoer.user().username : null // Reactive
  onData(null, {title: userName})
}

export default composeWithTracker(compose)(HelloWorld2)

there's a typo in line 6 above:

let userName = Metoer.user() ? Metoer.user().username : null // Reactive