To make People picker readonly, you can use the below JQuery code:
$(".sp-peoplepicker-delImage").css({ 'display' : 'none'});
$(".sp-peoplepicker-editorInput").css({ 'display' : 'none'});
You can also apply them with the help of css:
<style>
.sp-peoplepicker-delImage{
display:none;
}
.sp-peoplepicker-editorInput{
display:none;
}
</style>
This is the easiest and fastest way to make people picker fields read only in SharePoint
2013/online, but it will make every people picker field on the form read only. So please let
me know if you want for a specific column.