It depends. Would you extend the same suggestion to other potentially "undesirable" pax in C class? Including, but not limited to:
-drunken, snoring "businessmen"
-IT consultants who are constantly getting stuff out of the overhead
-elderly VFR who seemingly go to the lavatory every 20 minutes, pulling as hard on the seat backs as possible to make their way down the aisle
-first time business class gigglers who can't stop taking selfies and wandering all over the cabin
-op ups or upgrades for any reason that don't involve a cash payment

Hmmm....the airline is willing to offer them a seat in a particular class on their plane. So, I guess it's still rude to suggest it to anyone who has been seated in C by the airline.