Wavy shape with css

I think this is the right way to make a shape like you want. By using the SVG possibilities, and an container to keep the shape responsive.

svg {
  display: inline-block;
  position: absolute;
  top: 0;
  left: 0;
}
.container {
  display: inline-block;
  position: relative;
  width: 100%;
  padding-bottom: 100%;
  vertical-align: middle;
  overflow: hidden;
}

<div class="container">
  <svg viewBox="0 0 500 500" preserveAspectRatio="xMinYMin meet">
    <path d="M0,100 C150,200 350,0 500,100 L500,00 L0,0 Z" style="stroke: none; fill:red;"></path>
  </svg>
</div>

I'm not sure it's your shape but it's close - you can play with the values:

https://jsfiddle.net/7fjSc/9/

#wave {
  position: relative;
  height: 70px;
  width: 600px;
  background: #e0efe3;
}
#wave:before {
  content: "";
  display: block;
  position: absolute;
  border-radius: 100% 50%;
  width: 340px;
  height: 80px;
  background-color: white;
  right: -5px;
  top: 40px;
}
#wave:after {
  content: "";
  display: block;
  position: absolute;
  border-radius: 100% 50%;
  width: 300px;
  height: 70px;
  background-color: #e0efe3;
  left: 0;
  top: 27px;
}

<div id="wave"></div>

My implementation uses the svg element in html and I also made a generator for making the wave you want:

https://smooth.ie/blogs/news/svg-wavey-transitions-between-sections

<div style="height: 150px; overflow: hidden;">
  <svg viewBox="0 0 500 150" preserveAspectRatio="none" style="height: 100%; width: 100%;">
    <path d="M0.00,92.27 C216.83,192.92 304.30,8.39 500.00,109.03 L500.00,0.00 L0.00,0.00 Z" style="stroke: none;fill: #e1efe3;"></path>
  </svg>
</div>

https://jsfiddle.net/1b8L7nax/5/

Recently an awesome tool called Get Waves was introduced where you can simply from UI create your own waves and then export this to SVG format. This is as simple as going to the https://getwaves.io/ website and enjoying it!

Edit:

Recently I've discovered also a new tool - https://shapedivider.app/

Edit2:

Yet another, this one looks advanced with the points when to break curves https://www.svgwaves.io