Comment in Microsoft List

You could create a separate "Comment Count" column and use this JSON formatting to show if their are comments or not. That would allow to still apply your other formatting to the Title column. Not a perfect solution but may help.

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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