No, I'd never heard of Lana Del Rey or Karmin before they made their appearance as Saturday Night Live's musical guest. But they're hardly the most obscure acts on SNL, either. Every once in a while, the show will peg its reputation on naming little-known talent, and sometimes it gets away with it: Natalie Imbruglia was on before she had a record out in the U.S. (March 7, 1998; Scott Wolf was the host), and of course "Torn" went on to be one of the biggest hit of the 1990s and is still somewhat inescapable on the radio today.
But sometimes they aren't so lucky:
Harlan Collins and Joyce Everson (May 29, 1976; Elliott Gould was the host) Folkie duo who, best as I can tell, never released an album or even a single. Collins was the longtime author of the syndicated newspaper feature "Today's Chuckle." No, I'm not making that up.
Richard Baskin (March 12, 1977; Sissy Spacek) Hot because he was the key arranger and sideman for Robert Altman's Nashville, he never released a record of his own. Spacek was making 3 Women with Altman at the time. Brother of staff photographer Edie Baskin, which probably didn't hurt.
Alan Price (April 23, 1977; Eric Idle) Former keyboardist for the Animals who had launched a solo career. His biggest American solo hit had been a cover of "I Put a Spell on You," which went to Number 80 on the Hot 100 back in 1966. Gets extra credit here because his performance was a godawful, lounge-worthy mess.
Libby Titus (October 15, 1977; Hugh Hefner) Another boring folkie. Wikipedia says she released "several solo albums," although the only one I can find evidence of is 1977's Libby Titus. Very well-connected in the music business; Carly Simon wrote a song about her, Donald Fagen married her, and Levon Helm had a son with her. [EDIT: As has been helpfully noted in the comments, it's actually a daughter, Amy.]
The Spanic Boys (May 12, 1990; Andrew Dice Clay) One of the fill-ins when Sinead O'Connor suddenly announced she wouldn't appear with Dice Clay, supposedly because of his misogyny but also, I hope, because he wasn't funny. Father-son roots band from Wisconsin; I actually saw them open once for the BoDeans. Opening acts for the BoDeans generally don't get to go on SNL.
The Tragically Hip (March 25, 1995; John Goodman) Huge in Canada, their biggest splash in the U.S. came with the 1996 album Trouble at the Henhouse, which peaked at Number 134 on the Billboard album charts. Fellow Canadian Dan Aykroyd is apparently a big fan; he introduced them on the show, among several guest spots he did that evening. So thoroughly Canadian they have both a "Gordon" and a "Gord" in the band.