The css property text-decoration-skip-ink: none; might help if the goal is to make the underline more visible. Text underline is normally covered by descenders, i.e., parts of the letter that drop below the line. The skip-ink property allows the wavy underline to cover the text. It seems to have good browser support. And here is a link to documentation.

The Monaco editor is probably not a good solution if all you want to do is underline some text.

Run the code snippet below for examples of with and without text-decoration-skip-ink.

.wavy1 {
  text-decoration: underline wavy red;
}

.wavy2 {
 text-decoration: underline wavy red;
 text-decoration-skip-ink: none;
}

<div style="font-size:2em;">

  fg(23<a class="wavy1">,</a>)  fg(23<a class="wavy2">,</a>)
  <br/>
  <span class="wavy1">jgpyq</span> <span class="wavy2">jgpyq</span>

</div>

Building on @Roberto's answer, you could use the following CSS to insert content after .myInlineDecoration, set the content itself to transparent and adding an underline to it (which is visible):

.myInlineDecoration {
   text-decoration: none; 
   position: relative; 
 }   

.myInlineDecoration:after {
    text-decoration: underline;
    text-decoration-color: red;
    text-decoration-style: wavy;
    position: absolute;
    height: 1px;
    margin: 0 5px; /* adjust to move left and right */
    content: 'x';
    color: rgba(0, 0, 0, 0);
    left: 0;
    bottom: 13px; /* adjust to move up and down */
}

The Javascript is as follows:

var editor = monaco.editor.create(document.getElementById('container'), {
    value: "fg(23,)\ngh(45,)",
    language: 'javascript'
});

var decorations = editor.deltaDecorations(
    [],
    [
        {
            range: new monaco.Range(1, 6, 1, 7),
            options: { inlineClassName: 'myInlineDecoration' }
        },
        {
            range: new monaco.Range(2, 3, 2, 4),
            options: { inlineClassName: 'myInlineDecoration' }
        }
    ]
);

I'm not convinced it's the best solution (feels quite hacky using CSS rather than relying on Monaco - but then I'm not sure how to do it solely in Monaco)