A poll surveying American attitudes towards the coming GOP takeover of the House makes it very clear that the Republicans do not carry with them a "mandate". In fact, they asked that very question, and only 17% of Americans thought the elections constituted a mandate from the American people.
Rather, Americans voted in Republicans because they were frustrated with Democrats, and wanted a change. Not that they expect much -- 64% think Republicans will do as well or worse.
So it's basically a classic mid-term in a poor economy. Change for change's sake. It's not an illegitimate opinion by any means. But it's not a mandate.