It is not possible to diff 2 JSON contents in the 'content differ'.
Add the following contribution in the right contribution file:
<extension target="org.nuxeo.ecm.diff.content.adapter.ContentDiffAdapterManagerComponent" point="mimeTypeContentDiffer"> <contentDiffer class="org.nuxeo.ecm.diff.content.adapter.HtmlContentDiffer"> <pattern>application/json</pattern> </contentDiffer> </extension>