Jim Bakker made a pretty good run at the title, for someone not in politics.
In the '80s, Jim and Tammy had 10+ million viewers to their TV show, and ran Heritage USA, the 3rd largest amusement park in the US.
A couple of years later, he was in prision, found guilty of 24 counts of mail fraud, wire fraud and conspiracy. Hmm, I hadn’t know that he shared a cell with Lyndon LaRouche. Bet that lead to some interesting conversations.
The OP specified people, but, as you alluded, it could be successfully argued that the largest collapse of relevance/respect in the past 100 years was that of the USSR (or, to go wider, the entire European Communist bloc) from 1989-1992. In 1988 nobody expected European Communism to be peacefully swept away in a tide of indifference and exhaustion, but that’s pretty much what happened.
How about Johnny Carson? He was such a universally admired figure as host of the Tonight Show for so many years. When he retired he deliberately shunned the public eye (I think he appeared in a random Letterman skit once). So, in terms of the “importance” category I think he rates a mention.
Burying all that gold under Stan Smith’s house probably didn’t help either. 
Clinton?
Otara
“Took industrial action”?
Since the OP did specify people, I’ll go for a variation on what you said and say “Karl Marx”. His reputation in much of the world went from being a near deified prophet of the inevitable course of history to being the progenitor of a disastrous failed movement.
He’s done pretty well for himself since 2004, he certainly dropped in relevance after '04, but I don’t think he really “cratered”.
John Edwards was more or less on a slow decline prior to his sex-scandal, he went from VP candidate in '04 to “also ran” status after '08, I doubt he would’ve been much of a power even if he had kept his libido in check.
Emperor Hirohito dropped from being a deity when Japan surrenderd in WWII to just being a run of the mill symbolic monarch. Thats a pretty tall height to come down from.
I believe that is british english for “went on strike”
Hitler.
Started as a kooky chieftain who failed to overthrow the government to having one of the most powerful and feared militaries in the world. He was the uncontested leader of all affairs in Germany and was either adored or dreaded by anyone under his control. With better strategy, he might have been able to take out Russia and have access to massive natural resources. With better diplomacy, he might have delayed the USA’s entry into the war. Some more time spent on research and he would have had nukzis. He had a shot at making the whole world in his image.
Now teenagers make him say funny stuff in captioned videos on Youtube.
The German government is deathly afraid of looking even possibly aggressive in international affairs. It financially helps Israel whose creation Hitler is largely responsible for. Europe was split for more than 40 years between liberal democracies and communists. He killed himself in a bunker as the Red Army was approaching.
Well, in the allied countries were making fun of Hitler pretty much as soon as he came to power. Granted his respect in Germany is a lot less then it used to be, but even that was over the span of years as Germany’s fortunes in the war went from good to bad to doomed.
It is true that he was not respected by all, not even in the sense of fearful respect.
He still had plenty of relevance and importance, though. Perhaps the most important man of the 20th century in the same way Ben Laden was arguably the most important man of 2001.
Barack Obama.
Obama, nomination acceptance speech, June 2008:
Evan Thomas, editor of Newsweek, June 2009:
Mark Halperin, Time magazine, October 11, 2010 http://www.time.com/time/politics/article/0,8599,2024718,00.html#ixzz12CPY0CsK
Wishful thinking.
The first thing that came to my mind was OJ Simpson, just in terms of public mage.
Oliver North was a speaker at, and point of promotion for, this past summer’s NRA conference in Charlotte, along with Sarah Palin, Glenn Beck, Newt Gingrich and Chuck Norris. People like Haley Barbour and John Thune were there too, but I think North was a bigger draw than them.
John DeLorean went from big wig designer at GM, responsible for developing the GTO and Firebird, to leaving GM to create his own car (the DeLorean, of course) to trying to support his struggling car company by smuggling cocaine to closing up shop within a couple years of opening the doors. The car is somewhat of a collectible now, but if not for Michael J. Fox, nobody would probably care about it.
:::chuckling at image of DeLorean carefully poring over drawings of toupees::::
I’d say Colin Powell following his presentation on WMD to the UN if we restrict it to the past 10 years or so. In fact his entire reputation was what made him the perfect choice for the Administration of the time. Previous to that he was a perennial potential running mate to various people and even mentioned as the first black president.
He’s now seen as a compromised naif that was led down the garden path.
Yeah, excellent choice. He’s trying to repair his image by making mea culpas and working on urban education issues, but yeah. He was the one credible voice in the Bush cabinet - because of him, I supported the invasion of Iraq - and to me, his complicity in starting the war is equally disappointing and tragic. I think it’s fair to say that both Dems and Republicans respected Powell before 2001. Now I don’t think either party really gives him the time of day.
George W. Bush probably deserves to be mentioned here. Leader of the free world, demonstrated some concept of leadership post-9/11 (standing on the rubble at Ground Zero), victorious on the aircraft deck: “mission accomplished.” Has anyone seen this guy since he left the White House? His dad and Clinton are players on the international and philanthropy scene, Carter is doing his thing… I’ve only literally seen him on SportsCenter at a Cowboys game. I know people will say they didn’t respect him in the first place, but I wonder if that guy can even get a table at an RNC event? The GOP is the party of tea partiers and Palin - they’ve undergone a de-Bushification themselves.
Pretty amazing, as the incumbent in the White House is receiving massive abuse regarding the state of the economy, war effort, unemployment, etc., and a substantial proportion of the US populace seems to forget these are all situations created by the previous officeholder…
Specifically, Cardinal Law of Boston. He went from being a respected church leader to a an unindicted felon. The man revealed (in questioning) an almost total lack of comprehension about what some of the priests in his diocese were up to (child molestation). He was made aware of some terrible activity (by a priest named John Geoghan), but did nothing about it. His solution for the activities of these pedophiles? Transfer them to another parish (where their evil deeds were unknown).
As far as I know, Law has never expressed any remorse for his behavior.