Maggie Haberman and Ben Smith have a nice piece on the Donald Trump pseudo-candidacy. Long story short, his campaign is not a joke in the sense of Trump being in on it, but it is a joke in the sense that his entire career is a joke:

The widespread assumption that Trump’s flirtation with the presidency is a publicity stunt is no doubt at least partly true. But that’s merely the point of departure for a man for whom almost every public move over the past 30 years has been a publicity stunt. Trump has, in the past, hinted at presidential bids, only to pull back after basking in the public interest. But in the same voraciously media-hungry spirit in which he has leveled an array of accusations — some overstated, others flatly false — atPresident Obama in recent weeks, Trump appears likely to launch a formal presidential campaign, hire staff, shake hands in Iowa, participate in debates - in short, run for president. (See also: Trump draws Obama into 'birther' fray)
Trump is making the “types of moves that one makes if they’re actually running,” said one top Republican consultant familiar with his efforts.
The real estate mogul has spoken to pollster Tony Fabrizio as well as members of Larry Weitzner’s Jamestown Associates, sources said. Three Republicans said that Florida-based media consultant Rick Wilson had been recommended to Trump as a potential hire. (Wilson declined to comment on whether he’d been approached or spoken with Trump).
He recently called pollster and strategist Kellyanne Conway, who agreed to set him up with some evangelical leaders.

Trump as a GOP candidate represents the easiest oppo hit in the history of primary races. He has advocated single-payer health care and called George W. Bush "evil." You can imagine a world in which he could win the nomination. That would be a world in which the Republican Party as an organized political force, as opposed to a collection of voters, ceases to exist. It would be like the end of the Roman Empire, when anybody who can grab some soldiers together can call himself the Emperor. But, while the GOP establishment may have lost some power, we don't live in anything resembling a world like that right now.