@ Yiyun _ Xiaoyin method is very correct, and there is another solution:

mounted , use this.editor , and do not assign editor.txt.html (this.quizData.quizTitle)

mounted() {
  this.editor = new E(this.$refs.editor)
  this.editor.customConfig.onchange = (html) =>{
    console.log(html)
    this.quizData.quizTitle = html
  }
  this.editor.customConfig.menus = [
    'head',  // 
  ]
  this.editor.create()
},

in this case, in methods , you can flexibly use this.editor assignment this.editor.txt.html () , emptying this.editor.txt.clear () and so on.

//methods
  getQuizInfo () {
    quizInfo().then(res=>{
      this.quizData = res.data
      this.editor.txt.html(this.quizData.quizTitle)   
      // this.editor.txt.html(this.res.data.quizTitle) 
    })
  },

here it seems that the server gets the data, which is asynchronous.

so the question is: when create sends a request to obtain data, mounted uses data directly, so how can you ensure that the data request is successful when you mounted? Your problem should be that there is no data when you assign mounted values, so it is invalid.

getting the data setting data should be a method, which should be called at the time of mounted to get the data and then assign the value.

mounted() {
  var editor = new E(this.$refs.editor)
  editor.customConfig.onchange = (html) =>{
    console.log(html)
    this.quizData.quizTitle = html
  }
  editor.customConfig.menus = [
    'head',  // 
  ]
  editor.create()
  this.quizInfo().then(res=>{
     this.quizData = res.data
     editor.txt.html(this.quizData.quizTitle)   
  })  
},

Thank you for asking, your question description solved my problem!