Or is it? If lots of people believe it, that in itself is serious.
I tried searching for statistics, and most numbers I found lacked citations. For example, this article claimed its numbers came from Pew Research Center, but failed to actually cite the survey. I couldn't find any such survey fitting the description. Also, the "return of Jesus Christ" is not equivalent to the Rapture. FAIL
But I eventually found a relevant study from 2006 produced by Pew Research Center (see page 19-22).
Graphic comes from Pew. The survey included 2,003 US adults, but the above graphic only shows statistics for the 1,670 respondents who identified as Christians. [Edit: clarified]
Unfortunately, the survey did not ask about belief in the Rapture, much less Rapture specifically on May 21st. You can't always get what you want. I'm not sure, but I think the best proxy is the belief that the world situation will worsen before the second coming. Pew identifies this belief with "pre-millenialism", which is one of the varieties of Rapture beliefs.
Are these numbers higher or lower than you expected?