Difficult to think of a situation in which a teacher's sarcasm
would "lighten the mood" in any classroom.
Federal law says that every classroom needs to be a safe place
for the children. Sarcasm is generally mean-spirited; the word
literally means "stripping away the flesh." Have never seen
this issue mentioned in any textbook, but in general, teachers
should avoid any form of communication that might make the
children in the room feel uncomfortable or threatened.
So it's a bit disturbing that you've run across many different
"professionals" who feel that sarcasm is an acceptable way of
communicating with their students. Sounds like you've had the
bad fortune to encounter a lot of teachers who fancy themselves
stand-up comedians. The classroom is all about the children's
needs, not about any teacher's inner desire to bump up his own
ego at the students' expense.