Some passengers leave their brains in the airport terminal, we all know this. They sometimes lose a fraction of the social etiquette they would have in their normal environment.
Passengers often make personal comments to try and make a common ground with the crew, in case they need them later, or just to be polite in not ignoring them, but can often overstep the mark without realising, especially if the crew member is very sensitive.
Cabin crew are there to think and act for all those who aren't thinking normally for themselves, just look at the wording in your drills - passengers need to be told to come towards the door to get out.
Look at the amount of times you make a simple request like to fasten their lap strap, and are looked at like you have just asked someone to tattoo your left nipple. Sound familiar?
Okay so someone has just made a comment that your teeth look bad. It is possible they are (or live with) a dentist, or have a friend who has just had similar work done on their teeth and the result was so impressive they think they are being helpful to you.
They probably wouldnt say it to someone on the street, or maybe they are one of the more outspoken members of the public, but passengers see crew as exceptionally friendly and approachable so they feel they can share almost anything with you.
Are you Jewish?? I wouldn't find that insulting! Maybe they were Jewish and as some religious followings are very proud of their faith, maybe they thought you would be pleased to be complimented. How often do you instantly warm to someone whos wearing the shirt of your favourite team or city, etc....?
In any customer service role, you are representing your company. Not yourself. If you find a comment odd, don't answer back as they may feel their comment is totally innocent, and it will come back on your employer in the long run. Humour and sarcastic remarks arent taken the same by everyone.
If it is a threat to either yourself, or the aircraft, then yes take it seriously.
Sydgirls pax who said she looked like a girl he once f***** in high school, in my head I'd be thinking "well clearly youre mistaken as I'm still smiling" or "ouch" or "actually I'm a virgin you as***le", but I wouldnt say it, i'd either ignore them and smile or just smile sweetly and say nothing or say, "really? aw well it wasnt me".
Only when I feel threatened or very humiliated would I politely say something.