Should teachers be entertainers?
Teachers and professors don't need to be clowns. However, they do need to engage their students.

The old mode of a teacher standing at the front of the lecture hall talking "at" students for an hour or ninety minutes at a time is a cultural relic, rather than necessarily the best way to educate students, as learning in a lecture is usually at best passive. 

There are many ways to get around this problem—including engaging in class discussions, demonstrations, group learning, and, yes, sometimes humor can be used to make the point. But just as teaching should not be a stale recitation of facts, neither is it open mic night at your local comedy club. Learning is serious business, but it doesn't have to be boring, either.