Is it wrong to say that my mother is a middle-aged woman under 40 years old?

This sentence does not repeat the amount of information, so it is not a bad sentence. If you insist on changing it, you can change it to: My mother is a housewife under 40 years old, or a white-collar company under 40 years old. In this way, the amount of information will appear to be more sufficient. What the poster posted is just a joke. If a teacher really thinks it is a bad sentence, it only shows that the level of education is limited.