WordPress comment form is usually placed at the end of each blog post where blog post readers can provide their feedback about the post, send in their queries and interact with the post author. As the comment is posted and approved by the wordpress site admin, its shown at the end of the blog post in the comment area. Recent Comments Widget (legacy) / Latest Comments Widget can be used by the wordpress admins to automatically list the most recent comments in their site’s sidebar. We will see different ways in which you can add the comment widget to the sidebar. This blog post can also be viewed as Recent Comments widget vs. Latest Comments widget as I have shown both of the widgets and how it looks on a wordpress site.
I am currently using WordPress version 5.9 and also using a plugin called Classic Widgets. When you install and activate Classic Widgets plugin, the Widget page view under Appreance is shown in old classic style where in you can add Recent Comments Block to the Sidebar by drag and drop.
Recent Comments block is a legacy widget which was available in the old versions of WordPress but if you still want to use recent comments widget then Classic Widgets Plugin should be installed and activated first. Instead of Recent comments block you can also use Latest Comments widget which we will see in the next sections of this blog post.
Recent Comments Widget

After you drag and drop Recent comments block to the right sidebar and click on Save button. This will display as a widget in the sidebar and show the users name and post link. You can adjust the number of comments displayed in the widget.
As we have just 3 comments for now, it shows only three comments. But this widget provides very basic information and does not display actual comment, comment date and also does not display User’s avatar. You can see below screenshot and check how it looks on the sidebar.

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Latest Comments Widget
Disable Classic Widgets plugin (If you do not use this plugin, then skip this step).

Click on Appreance on the left hand side of WordPress admin dashboard and then click on Widgets.
On the Widgets editing area click on + Sign and then search for Latest Comments block. Select Latest Comments block and add it to the sidebar.

You can adjust the Number of comments. Enable / Disable Avatar, Display Date, Display excerpt of Latest Comments block as per your requirement. I will recomment to keep this switched on to display information about the comment.

Once its added you will see that it shows the comment, comment date and user’s avatar and also looks nicer than Recent Comment Widget.

You will notice that there is no Title /heading of the Latest Comments block in Right Sidebar and there is no option to add it via Widgets editing area, therefore I have workaround it by temporarily enabling Custom Widget Plugin again and then manually inserting a heading html tag to add the title.
Enable Classic Widgets Plugin.

Go to Appreance -> Widget and Under the Right Sidebar find the Block for latest comments. Add below line before <!–wp:latest-comments /–>. You can customize font size, font family, add a background color via <h> heading tag or you can also use <p> tag if you want and style Recent Comments heading as per your requirement.
<h2 style="font-family:open sans;font-size:120%">Recent Comments</h2>

Recent Comments Heading has been added to the latest comments block in the Right Sidebar.

You do not have to keep Custom Widgets Plugin Activated, you can deactivate it once you added the title / heading of Latest Comments block.
Conclusion
If you want to see classic WordPress widgets settings screens and use Recent Comments Plugin then install and activate Classic Widgets plugin. However, Latest Comments Widget provides better view of comments with more display options like Date of the comment, comment text, User Avatar field etc. which can be enabled / disabled as per requirement. Therefore, I will recomment to use WordPress Default Latest Comments Widget and place it in your sidebar.