Are they supposed to be Earthlings?

Yes. Hillbillies, actually, going back to 1916 or whenever the strip wasa first done. The series discussed started in 1963 with King Features Syndicate, the mighty comic strip distributor, and Paramount/Famous/Harveytoons cartoons doing a couple of theatrical cartoons following a story arc where Snuffy, wife Louweezy, and Barney, in the city where Snuffy Smith snags a recording career..Snuffy's (Paul Frees)'s theme song and the song he sings here, is "Great Ball of Fire"(not the 1957 Jerry Lee Lewis song). Mishaps leading to a happy ending follow for him, Bareny Google (also Frees), and Lou-weezy aka Louisa (GeGe Pearson).

This led to the long running television cartoon which also had othert hit KFS cartoons: Mort Walker's Beetle Bailey (Howard Morris), which costarred long suffering superiors Sargent Snorkle (Allan Melvin) and General Halftrack (Howard Morris), (pretty recent at that time, being a 1950 creation), and another older, World War I entry, Geo.Herriman's "Krazy kat", with the masochistic, love struck girl Southern cat Krazy being hit with a brick by Ignatz, then winding up trying to get Offisa Pup, the ever present anthromorhpic dog to release her own assailant! (Krazy=Penny Phillips, Ignatz and Offisa Pup,= both Paul Frees again). Unlike Barney/Snuffy, a handful of studios (Famous as mentioned, best known for the Harvey comic Casper) also did the Krazy Kat and Beetles as well, including Bozo producer Larry Harmon.