Partisanship aside, what makes you think so? I'm really disappointed with Obama (though for reasons which I would think would make you like him more -- on almost everything important he's governed either center* or even center-right), but I wouldn't call him a "joke." IMHO Dubya was a joke (albeit one in very bad taste) and Quayle was, each in the sense of being obvious intellectual lightweights who probably shouldn't even have been trusted with running a corner store let alone the country, but other than that I wouldn't call any President or VP since the 19th century a joke. (FWIW I wouldn't call Romney a joke either; Gingrich I don't think is a joke, he's the sort of Grand Guignol candidate that would clean up in virtually any Gubernatorial race -- I can't see him as president but, meh, we've had far worse.)
* thinking about it, maybe that's not quite correct. What Obama really has been is the consummate Establishment president, like Bush Senior was or, better example, Gerald Ford. His governance isn't left or right, it's just Status Quo. It's particularly galling because he ran as the Anti-Establishment candidate -- everybody does but he even more than usual. It's as if George McGovern had been elected and morphed into Nixon right there on the dais as he took the oath.