Some students giggle at me while I teach!
In my experience, there are some students who just cannot take class seriously. They may not even be laughing at you. Rather, they may be talking with a classmate and find what they are discussing amusing. They may not be paying attention to you at all. When they take a test and do not do well, they may react by thinking that is funny. If it bothers you enough, you should talk with the students after class and explain that their behavior is distracting to you, and probably to their classmates. This might appeal to whatever sense of fairness they may have. If they continue to be a distraction for you, a pointed remark about immature behavior may be appropriate, again after class. You may, if you wish, indicate that their behavior shows a lack of respect for you, the course, and their classmates.

I would not call the students out in class. I would also consult the department chair for suggestions on how to handle the situation.