Thanks, it works fine.

As all my fields are required I ended with a similar solution with a loop:

mounted() {
    const labels = this.$el.getElementsByTagName('label');
    for (let item of labels) {
        item.innerHTML += `<sup class="text-danger ml-1">*</sup>`
    }
}

It doesn't work, because the prop's value is rendered with interpolation, not the v-html directive. As it's not standard behaviour, you need to use a walkaround:

<mdb-input type="text" label="" v-model="inputValue" ref="customLabelInput"/> 

mounted() {
    const [ label] = this.$refs.customLabelInput.$el.getElementsByTagName('label');
    label.innerHTML = `Your name<sup class="text-danger">*</sup>`
  }

Best regards, Magdalena