Your flight attendant might look tired but never tell him or her they look tired. Your intention might be well-intended as you might be trying to be friendly and strike up a conversation but this is a complete No No.
It’s generally considered polite not to make unsolicited comments about someone’s appearance, especially if it could be interpreted as negative. Telling someone they look tired might be perceived as pointing out a flaw.
Flight attendants are providing a professional service. The job can be physically and mentally demanding. They often deal with long hours, different time zones, and challenging passengers. Commenting on their tiredness may be seen as questioning their ability to perform their duties.
Top Tip: If you want to strike up a conversation with a flight attendant it’s far better to show empathy towards them. You can empathise with our tough their job is and you don’t know how they do it and then there is all the patience they require to do their job.