Yes, if you watch too much tv with little rest for the eyes. Your eye muscles will get tired which cause the eyes get short sighted. The distance of the pupil will be longer. Every 300 degree prescription will cause the 1 mm longer distance of pupils. That is why the square eyes appear.
My dear clever boy/girl, what an interesting question, and what an interested mammy of yours. You know when you are watching TV, the TV is square, and so its reflection in your eyes is square. Then you could check yourself if your eyes are square or not. Anyway, have you find a person on the street with square eyes? If have, your mother's opinion will be absolutely right!
No, you will not get square eyes. It is just a saying that adults say to their children, hoping will not watch too much TV. In fact, parents are good for you. Watching too much TV will not make your eyes square, but it will hurt your eyes. It can make your eyes hurt if you often watch TV for a long time without a break. And it can weaken your eyesight. So do as your mother told you, even if you know you will not get square eyes.