Is it rude to forward an email message to the original recipient if they did not respond?
On the whole, I'd say "yes, this is a rude way to send a reminder to a non-peer."

The premises are tricky: what is "prompt" to one person is not to another, and not everyone checks (work-related) email at every available opportunity. E.g., some seem to only go through email once a day, on weekdays, or even less often.

There is the other issue about acknowledgement of receipt, even if one's response will be delayed.

But/and if one is teaching a course with 30 or 100 or 250 students, individual responses to daily can be a time-consuming occupation... so that "acknowledgement of receipt" becomes unappealing and an arguable waste of time... if a proper understanding can be reached.

I've tried to cultivate in my students a sense that emails should have a "greeting", a "body", and a "closing", much as old-timey paper letters did... if only so that one can tell whether a thing was mis-sent and/or truncated, not to mention some overt acknowledgement of the identity and dignity of the recipient.

So, if I were to receive a fowarded copy of an email that had already been sent to me, I would consider it quite rude, yes. I do attempt to respond within a few hours of receipt of all email, even if only to acknowledge receipt. Whether or not I had done so, I'd view a merely-forwarded email as uncivil, apart from issues about peer-or-not.