The comments are stored in the ApproverComments field/column of the task so what you'd need to do is query that task list.
When you use something like a flexi task, you can store the task ID in a variable
Once you have the ID you can use the query list action to query the task list that you've saved the task and get the contents of the ApproverComments field. Keep that it will store everything from line 2 onwards of the email so you might get more than just their comments - you'll get the original email text and their email signature etc too. You could use a regex or something to try and get rid of the unwanted text.