Time and place...
I've had it at a full service airline, on a large aircraft on an 9hour flight that had been very busy. I thought it was appropriate, the passengers could see we had been doing our best in trying circumstances.
I've also had it at a short haul lcc, on a smaller aircraft after a 4 day trip with the same crew. After 2 medical emergencies on the last flight (a red eye) and add to that a disruptive pax, it was appropriate.
Have had a few flights where the passengers themselves have initiated '3 cheers for the crew!'.
I dont see it appropriate on a normal day to day flight where you just do your job... Do it in the galley on descent, or on the crew bus.