Personally I don't understand why the attractiveness or gender of flight attendants is so important to people.
For me as long as they're polite and clean I don't care who is my flight attendant.
The gender is something old, back in the day pilots where males and the attendants where female. Stuck around for a long time as it was seen un-manly to be a flight attendant. And some people are just morons that never got over the idea that those gender roles are gone.
Attractiveness is relative, the airlines don't really care about attractiveness itself. But they do have a catalog of standards for their attendants.
Among them are
height/reach - Too small and you can't reach the upper cupboards or the hand-luggage. Too tall and and in certain planes you run into trouble. The optimal reach is about 2 meters.
Weight - You just can't use obese people, the space to move is limited in planes and nobody wants a flight attendant that bumps into passengers while serving drinks. Also too thin and you're facing a whole different set of troubles.
Likewise other standards for appearance are asked depending on the airline. No visible tattoos, piercings, natural hair colors, clean shaven men. Usually they want flight attendants that won't "insult" anyone. So neutral, clean appearance is best.
But we are far away of the ancient times where a perfect flight attendant was an ex-model.
The overall appearance demands are about the same as in any higher class restaurant.