To be fair, is it a joke for master’s students to do sales?

Impossible. There are professional masters in the sales industry. If you can do sales with an ordinary master's degree, it is considered very good. Of course, this refers to the sales that make money, and the sales that don't make money, four or five thousand a month, that's a joke.

In addition, you, a graduate student in engineering and science, work in sales. It seems that your major is not suitable. It may be a waste of your professional knowledge, but it does not waste your livelihood skills. Your research time may be wasted, but your life is not wasted.