<div class="container">
  <div class="inner">
    <div class="contents-wrapper">
      <p class="text">mix-blend-modeが反映されていない</p>
      <img src="https://drive.google.com/uc?export=view&id=1VDIZg2YmeyoJaLbwhlbiAm7f_3u8bT0L" alt="">
    </div>
  </div>
</div>

* {
  box-sizing: border-box;
  margin: 0;
  padding:0;
}

html,
body{
    height: 100%;
}

.container {
  display: flex;
  justify-content: center;
  background: #5b7e91;
}

@media screen and (max-width: 480px) { 
  .container {
    height: 80%;
    }
}

.inner {
  position: relative;
  transform: rotate(5deg);
  height: 300px;
  width: 80%;
  display: flex;
  justify-content: center;
}

.contents-wrapper {
  position: absolute;
  left: 50%;
  transform: translateX(-50%);
  width: 80%;
  text-align: center;
  mix-blend-mode: overlay;
}

.text {
  display: inline-block;
  margin: 1em 0;
  color: #fff;
  font-size: 1.3em;
}

img {
  width: 100%;
}