From metafandom on LJ: Living in a "working poor" background, I don't think "The Bill Cosby Show" counts. They "code" middle-to-upper class to me, and I'm pretty sure the parents are both professionals. And I'm pretty sure it's a sitcom, laugh-track or no.
I'm honestly not surprised about the results, though, at least from the US side of things (the British can explain for themselves): First, class isn't talked about in the same way, say, race or gender are, so it's less likely to be the background of a show, unless (as with "All in the Family" or "Roseanne") the writers/producers come from that background and know what the issues actually are. (Count the writers/producers from working class backgrounds in Hollywood.) Second, being present but not discussed generally means that it's touchy, and the default for touchy issues is generally comedy or sci-fi. (Try looking for serious dramas about race or gender in the 60s, and then compare with the number of sitcoms and sci-fi shows that deal with those issues.)