<template>
  <div id="counter">
    Counter: {{ counter }}
  </div>
</template>

<script>
export default {
  name: 'HelloWorld',
  data() {
      return {
        counter: 0
      }
    },
  props: {
    msg: String
  }
  }
</script>

这样可以：

<template>
  <div id="counter">
    Counter: {{ counter }}
    <h1>{{msg}}</h1>
  </div>
</template>

<script>
export default {
  name: 'HelloWorld',
  data() {
      return {
        counter: 0
      }
    },
  props: {
    msg: String
  }
  }
</script>

加了一行这个

{{msg}}

。
如果在app.vue中行不行？
试过，也行。

<template>
  <HelloWorld msg="Welcome to Your Vue.js App"/>
  <h1>{{msg}}</h1>
</template>

<script>
import HelloWorld from './components/HelloWorld.vue'

export default {
  name: 'App',
  components: {
    HelloWorld
  }
}
</script>

<style>
#app {
  font-family: Avenir, Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  margin-top: 60px;
}
</style>