It is out right lashon hara to criticize and make fun of a drasha/lecture or sermon said by anyone especially a rabbi. Assuming what was said was not anti-Torah even if it is true that he might have made some mistakes it is still lashon hara even if it is true. Everyone likes different styles, some people like hearing stories, others like hearing new insights into the verses of Tanach, and even others might enjoy a deep lumdishe shiur in halacha therefore no one can complain to others that the sermon was boring or irrelevant because one person might have interests in something said whereas the other didn’t. Lessons hit home with different people in different ways therefore even if the complaint was b’apei tlasa, in-front of at least 3 people so word spreads it is still not permissible since everyone has their own opinion about these types of matters. Especially since many times it is guaranteed that things will be exaggerated and misquoted or half quoted so there is definitely no room to even think that the leniency of apei tlasa can be applied here.
What is worse is that these type of people who are scoffers and ba’alei lashon hara purposefully run to these sermons just to make fun of them afterwards with their group of friends. Not only do they get a sin for speaking lashon hara, scoffing at sages and words of Torah and lying, but the very steps they take to come to shul to hear the sermon are each a sin since they are running to do a sin. Not only that but often when the sermon is done there is a Kaddish and they are already gossiping with their friends so they miss out and cause others to miss out on amens and yehei shmei rabba which is worth keeping the whole world in existence. These scoffers make fun of the rabbi that he imperfect or he is just doing it for the money so why should we listen to his rebuke and mussar.
It takes one to know one, the real issue is that because they know they have faults which should be corrected, the natural tendency of a human being is to be on the defensive. Even if it is true that the rabbi goes around saying lectures to support his family, he still most likely chose this profession because of his love of Hashem and his brethren who he wants to see doing the right thing, therefore there is no room what so ever for any joking around and belittling of Torah lectures and sermons, especially the Rav of a shul who people constantly look up to for guidance in Jewish law and general advice. It is a tremendous chillul Hashem that could cause a lot of monetary damage and embarrassment.