Per the documentation: https://docops.ca.com/ca-performance-management/3-0/en/create-custom-attributes-354253407.html we note the following:
Specify whether to hide the property by default from the inventory views in the Performance Center user interface. If this tag is set to "true," you can manually choose to display the custom attribute in the user interface. This tag impacts whether custom attributes appear by default from the inventory views only. They are available by default in other areas.
To change the value to false you can do the following in a REST client:
After doing this the custom attributes should now be visible in the CAPC user interface