IDIOMS

but seriously folks

(idiomatic) Directs attention to immediately preceding failed attempt at humor.

Example: 1969, Julius J. Epstein, But, Seriously ...: A Play in Two Acts, page 18
  A nightclub comic who gets off a bad joke, nobody laughs, but he goes right on with: "But, seriously, folks" […].
2004, Gerald Nachman, Seriously Funny: The Rebel Comedians of the 1950s and 1960s, page 23
  But seriously, folks, on behalf of the Copacabana — and believe me, I'd love to be half of the Copacabana
2006, Bob Mitchell, Match Made in Heaven, page 199
  "Take my life, … please," Colombo continued, a wry smile creasing his face. (Apparently he was a fan of Henny Youngman.) "Badaboom! But, seriously, folks, I learn long ago, during my life, that justice, she does not exist. […]."