The people who care that it's a Heroku issue not app issue (hint: not many) have probably already heard about the Heroku outage.
Everyone else will be confused about what the hell this "Heroku" thing is.
That said, if they don't use appropriate error codes (maybe 502 or 504 for Heroku issues and 503 for app issues?) they should. But I don't think error messages should mention "Heroku" by name.
I think that's true with Heroku, but gets less true as providers get bigger. If you tell a user that your service is temporarily down "because Google is down", even a lot of regular people will know what that means, and not really blame you for it.
Not necessarily true, especially if your customers are management types looking to lay on the blame as thick as possible wherever they find it, and will terminate a relationship if they believe the service-provider is incompetent.
For those kinds of customers, they may understand what Heroku is and why their vendor is using it and will definitely make at least some distinction about outage fault.
Everyone else will be confused about what the hell this "Heroku" thing is.
That said, if they don't use appropriate error codes (maybe 502 or 504 for Heroku issues and 503 for app issues?) they should. But I don't think error messages should mention "Heroku" by name.