Hilchos Lashon Hara, chapter 2, section 12

If someone gives a talk in a beit midrash, it is forbidden by halacha to make fun of the speaker and say the talk had nothing in it and was not worth hearing.

Sadly, many people are careless about this and do not think such mockery is forbidden at all. But by halacha, this is real lashon hara. It causes the speaker loss of income, and also pain and embarrassment. And even if it is true – lashon hara is forbidden even when it is true.

If a person truly cares about the speaker, they should give advice privately and point out the mistakes. By giving this advice, they also keep the mitzvah "You shall love your fellow as yourself." In any case, it is forbidden to make fun of the speaker in front of people.