Comment in Microsoft List

Jin Chen 

Try using below JSON code. It will bold the title text and show comments icon if there are any comments added.

{
  "$schema": "https://developer.microsoft.com/json-schemas/sp/column-formatting.schema.json",
  "elmType": "div",
  "attributes": {
    "class": "ms-fontColor-themePrimary ms-fontSize-m"
  },
  "children": [
    {
      "elmType": "span",
      "txtContent": "@currentField",
	  "style": {
        "font-weight": "bold"
      }
    },
	{
      "elmType": "span",
      "style": {
		"font-size": "16px",
        "margin-left": "10px",
		"display": "=if([$_CommentCount] == '0' , 'none' , 'block')"
      },
      "attributes": {
        "iconName": "Comment"
      }
    }
  ]
}
 

Microsoft documentation: Use column formatting to customize SharePoint 

Additional References:

1. SharePoint Online: All you need to know about Commenting in Lists 
2. How to Enable/Disable the commenting in SharePoint Online/Microsoft Lists 

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