Hello,
the purpose of cm:failedThumbnail is to mark files for which the generation of a thumbnail or preview failed, so that Alfresco does not endlessly retry it. You may remove it in an exported ACP as it is just a marker for internal functionality and does not convey business data. If the Alfresco you import the ACP into encounters similar problems, it will simply reapply that marker or a similar marker specific to the Alfresco version (if a marker is supported at all).
Regards
Axel