A “tin-pot dictator” is someone who is cheap or commonplace, with little political credibility. Tin pots came to symbolize in the 19th century something that is cheap and without much substance.
 
“Tin-pot dictator” was cited in print in 1926, when it referred to Italy’s Benito Mussolini (1883-1945). “Tin-pot dictatorship” was cited in print in 1938. The terms are still used, even though tin pots are much less commonplace items.
 
   
Wiktionary: tin-pot dictator
Etymology
From tin-pot, a cheap metal disposable container, and dictator
Noun
tin-pot dictator (plural tin-pot dictators)
1. An autocratic ruler with little political credibility, but with self-delusions of grandeur.
Usage notes
Although still used today, this is a pejorative term coined in the days of the colonial British Empire, and referred to the Victorian innovation of the tin-pot, a very cheap metal container, the forerunner of the modern tin can.
 
20 February 1926, Bakersfield (CA) Morning Echo, “A la Mussolini” (editorial), pg. 4, col. 1:
Today the world is afforded occasional opportunities to observe Italy’s tin-pot dictator in eruption, but we in the United States, and particularly in the western section of the country, are being given glimpses often of a would-be dictator, ...
 
Google Books
Far Eastern Affairs Pamphlets
Volumes 1-2
1938
Pg. 28:
When we are asked to include within the necessarily limited boundaries of democracy the brutal autocracy of Soviet Russia; the cold and calculating imperialism of the British and French empires; and the vertiginous machinations of the tin-pot dictatorships of Latin America, wherein hardly a single reigning ruler but got power by violence, the whole thing becomes absurd.
 
Old Fulton NY Post Cards
22 June 1939, New York (NY) Post, “Charles Laughton Refuses to Give Up a Single Pound of Flesh to Become the Hunchback of Notre Dame in His Next Picture” by Michel Mok, pg. 13, cols. 7-8:
After his return to Europe, he hopes to make a picture of the late Sir James M. Barrie’s “The Admirable Crichton.” He believes that the story of the efficient butler who, after a shipwreck, becomes the tin-pot dictator of the small island where he and his employers land, carries a striking application to certain present-day political conditions.
 
Google Books
The Future, If Any
By Robert Noble Golding
Chicago, IL: Lincoln Printing Company
1941
Pg. 105:
We are the only possible candidate, because the American language is not the same as the English, we have to race to push to the front, and we are the only nation which can furnish, to every race, administrators of that racial strain who have been brought up according to democratic principles and who, consequently, will not try to turn themselves into nobles or tin-pot dictators.
 
17 January 1942, The Times-Picayune (New Orleans, LA), “Gate Rate ‘Fiasco” Hit as Racivitch Promises Relief,” pg. 2, col. 2:
Attacking efforts to prevent him from distributing circulars in his behalf, Mr. Sessions said “those circulars are going to be distributed all over the city and that tin pot dictator in City Hall and none of his stooges are going to stop me from putting over my message to the people of New Orleans.”
(Cicero Sessions, a candidate for district attorney.—ed.)
 
15 March 1946, Seattle (WA) Times, “New U.S. Isolationism Hits Franco to Avoid Russ Issue” by Randolph Churchill, pg. 14, col. 7:
An example of the latter tendency is shown by the vehemence with which they attack little tin-pot dictators like Generalissimo Franco of Spain and Col. Juan Peron of Argentina as a substitute for devising any effective policy to resist the territorial encroachments of Soviet Russia and the fifth-column activities of international communism.
 
Google Books
The Ordeal of Gilbert Pinfold;
A Conversation Piece
By Evelyn Waugh
Boston, MA: Little, Brown
1957
Pg. 125:
The two Generals were incensed against General Franco and made free use of “tin-pot dictator,” “twopenny-halfpenny Hitler,” “priest-ridden puppet” and similar opprobrious epithets.
 
15 March 1962, Seattle (WA) Times, “Black Ruin May Be Just Ahead for Castro” by William S. White, pg. 9, col. 4:
Can there be too much doubt, then, that the gray days will be succeeded before too long by the black days of ruin and fall for this tin-pot dictatorship?
   
The Word Detective
July 2nd, 2011
Tinhorn
(...)
Another adjective, “tinpot,” also reflected the bad reputation of tin, in this case referring to cooking pots made of tin, widely considered inferior and poorly made. Thus “tinpot” became slang in the early 19th century meaning “cheap, without substance, inferior and contemptible” (“Mr. Taylor is a patriot in his little tin pot way,” 1838). This usage is today often seen in the phrase “tinpot dictator,” meaning an autocrat who governs in a grandiose and brutal fashion but wields little or no real power on the world stage.
 
The Independent Sentinel
Common Core Is Rewriting History Like a Tin Pot Dictatorship
February 7, 2014
By Sara Noble
Abraham Lincoln’s religion was Liberal, George Bush stole the election, social justice is the only justice, and the Electoral College is a mistake. That is what students are being taught in their math classes thanks to the federal government and a number of large corporations.