If you can't see the video, the couple looks like farmers from the Dust Bowl era. They're selling Model Ts. The following dialogue ensues:
DAISY: I smashed a mirror 'cuz I saw a woman in there that's crazy!
RICK: All right. Don't make me put you back up in the attic please.
DAISY: Don't put me up there!
RICK: Dammit, Daisy! I wish I had a more legitimate treatment option other than the attic, but that's just where medicine is at.
DAISY: I think I killed that Navajo girl.
RICK: What?! Why is this the first I'm hearing about this?
DAISY: No one will know she's in our root cellar.
Sure, it's a throwaway line in a throwaway skit. Nobody will think twice about it. And Daisy the speaker is crazy, so we can't take anything she says seriously.
But really, why use "Navajo girl" as a punchline? How is that funny? Why would a writer even think of that as a possible punchline?
In its infinitesimal way, this line contributes to the idea that Native people are inconsequential. That they're nothing but the butt of stereotypical humor. That we can laugh at their (imaginary) deaths because they're just, well, Indians.
Who's a famous murdered white girl? JonBenét Ramsey? Okay, suppose the line was "I think I killed that beauty-pageant girl in Boulder, Colorado"? Would that be equally funny? Or would it seem tasteless and cruel?
Navajo girls get killed all the time, so it's really not a joking matter. But SNL seems oblivious to this bit of prejudice. That's not surprising given all the times it's spoofed Indians in the past. Like many other TV shows, it's tone-deaf when it comes to Native people, culture, and history.