I think it is depends on each person's angle of view. In someone eyes, this is so attractive; But in other people's opinion, it is so disrespectful for them if someone look them with canthus.
If you ask me, i don't think it is a polite behavior. What's more, squint eyes shall be a eye problem not a attractive looking.
The squinty eyes are also the problems of the eyes which are not attractive in my opinion. If you have, you need to use the special eyeglasses to help you get the right vision at the right position. If you want to make your eyes get attractive, you could use the contact lenses or eye make up to get it.
I think that the person with squint eyes is kind of weird. In our common sense, nobody keeps squint eyes on things. In another word, it's a disease. If you realized that earlier, you need to see the doctor. Otherwise, the condition gets heavier, and you will lose your attractiveness any more.