Should I uninstall Mail incompatible plug in which are disabled/inactive after Mountain Lion upgrade?
That always happens when a new version of Mail comes out, the Herald dev needs some time in order to get the plugin compatible.

If you still want to use Herald, just ignore it and watch @heraldapp on Twitter, to be notified when the update is available.

If you don't want Herald anymore, uninstall it by using the uninstaller in the herald.zip download file.