I have the ServiceNow app installed on both my indexer and search head (and, add-on installed on the indexer). My SH has the indexer properly listed as a search peer, replication is successful, and I can search the SN indexes from the SH. However, the SN app on the SH is not populating with any data, it just has a lot of zeroes. Why would this be? The app properly populates with data on the indexer.
Installing the add-on on the SH fixed the issue. Not sure why you need the add-on installed for every server that the app is installed on; it would make much more sense to only need the add-on installed on the one server ingesting data (in this case the indexer).