Not just sitcoms, commercials too and they are all white males.
White males are the only group they can use. Can you imagine the ruckus if the part was taken by a female, or a male of any ethnicity other than European?
Mostly white guys, yes. But they do it to black guys too. Now, heterosexual men of any race are of course exempt from the dufus moniker, naturally.
Watch a few more commercials. The white man is always stupid. If there is no white male, it will be the white female who is incompetent. Minorities are the superior breed according to Hollywood and New York. It was bad before Obama but it is just getting worse. Makes you wonder how this nation got to be so great if we were all so stupid.