Opportunity Response records associated with Default Contact should be edited and updated to an actual Contact.
For more information about how these get created in your system, see Default Contacts.
The Customer Portal Report Card gives an A grade when you have a decrease of at least 5% from previous month (or none). The number counted is Opportunity Response records associated with Default Contact.
Our developers are looking into a tool to assign Default Contacts of Opportunity Responses. Keep a look out for that in a future release! In the meantime, assigning default contact opportunity responses is a manual process.
Here is what we recommend: