Or who they were. I asked the internet. The internet doesn't know. Perhaps it was Mark Twain. He seems to have said most of the clever things that were said in the era from which I assume the phrase to have come.
When is a dictator tin-pot? Only when they're not that good at dictating. The people who are really good at the job get called something else- "emperor" for instance. The gold standard dictators hardly need a title at all; their name alone suffices: Stalin, Mao.
To be a gold-standard dictator you really need to die in your bed- still causing awe. This definition would exclude Napoleon. So was Napoleon tinpot? Hardly. Perhaps Napoleon was something else altogether.
The image that comes to mind is of some over-dressed shouty little chap- like Adenoid Hinkle in Chaplin's The Great Dictator- but the smarter kind of dictator knows to avoid the trappings of tinpottery. Napoleon favoured a corporal's uniform. Hitler kept it simple too- eschewing the display that- for example- Goering went in for. If you're the main man everyone knows who you are- and your aura alone suffices. You don't need epaulets, medals and plumes to draw all eyes when you enter the room. Most contemporary dictators- tinpot or otherwise- favour the business suit or its cultural equivalent. Erdogan, for instance, presents himself as nobody in particular- just another neatly turned out, middle-aged baldy- but the world knows otherwise.
This isn't going anywhere in particular, just noodling around the subject. "Tinpot dictator" is currently being thrown at Boris Johnson for proroguing a parliament he can't be bothered with. Cromwell did something similar once- and Cromwell was gold-standard. Johnson hasn't been in power long enough for it to be clear whether he's gold-standard, tinpot or not really any kind of dictator at all. He has more of the aura of power than any British leader since Churchill- but it could well turn out to be illusory.