Common, yes, but hardly universal. I’m not having any trouble thinking of examples of standup comedians who do not seem to have much anger, either in their comedic personas or in their “real life.”
It’s not surprising at all to me that many standup comedians, or other onstage performers, would be introverts. Performing for/at people is such a different activity from interacting with people one-on-one or in small groups. As @raspberry_hunter said in another thread:
Why Johnny Carson’s introversion is perhaps surprising is not because of his role as standup comedian but because of his role as talk-show host who who sat around chatting easily with celebrity guests.