Among the attendees at Jackie Mason's Dec. 30 opening night show of Much Ado About Everything were legendary Yankee Phil Rizzuto, comedians Colin Quinn and Jerry Seinfeld. The latter two recently appeared on Broadway--Quinn at the Helen Hayes in Irish Wake; Seinfeld at the Broadhurst. Much Ado About Everything, Mason's fifth show on the Great White Way, features his personal brand of borscht-belt cantorial styled standup that generally deals with such subjects as politicians, urban annoyances, and differences between Jews and gentiles.