Kleopatra shows a "Show not certified certificates" button next to the filter drop-down if there are any certificates that are not fully valid. Clicking this button selects the "Not Validated Certificates" filter in the drop-down. This filter shows any not valid certificates including certificates that have expired or that have been revoked.
I don't think that's the intention of the button. The tool-tip of the button suggests that it wants to support the user to find certificate the user has not yet certified. Also showing expired or revoked certificates makes no sense because, unless those certificates belong to the user, they can do nothing to validate those certificates.
I think clicking the button should activate a filter that lists certificates that are "good" (i.e. not revoked, expired, etc.), but that are not fully valid.