I would direct you to the relevant passages in Ian Kershaw’s excellent (and, er, long) biography of Hitler.

To summarise briefly, though:

Hitler survived that period through a combination of incomes. He was afforded a small monthly allowance by the Austrian state by virtue of his by this time being an orphan, and not yet old enough to inherit his father’s savings (not that those were particularly great). He also had a relatively comfortably-off female relative who had lived with his sister and dying mother while Hitler was schlepping around Vienna, and it was from her that he extracted a great deal of money over a period of a few years under the guise of needing it for his artistic studies - she was not to know that he had, of course, engaged in no such studies. He eventually convinced his sister to let him have all of the orphan’s allowance that was supposed to be split between them, doubling that.

Together these monies were enough for him to survive for a while and eventually end up in the sort of Men’s Boarding House, where he fell in with a few petty criminals/men fallen on hard times, with whom he set up a small business painting watercolours for postcards/wall art etc, which sold quite well and afforded him a fairly steady income (though apparently he was very lazy and only painted when he literally needed the money, frustrating his compatriots).

It’s worth mentioning that he was incredibly chaste. He spent lots of his money on opera tickets, but he didn’t drink alcohol, visit brothels or gamble. He’s also reported to have eaten very little, such as bread and butter in the morning and nothing else. Whether this was necessity or something else, I don’t know. He was, needless to say, a strange man. He may well have spent time in Cafes without spending much money there. He seems to have taken up most of his time monologuing at/berating other customers about Viennese society, politics etc.

There was a revisionist biographical trend of Hitler where people were trying to paint him as a middle-class dandy who swanned around Vienna on family money. This is wide of the mark (as Kershaw explains in detail), though it’s fairly clear how people arrived at that assumption since, when his parents were alive, the family was well off for a provincial household, and he spent most of his time in cafes, at the opera and in museums. He also famously carried a cane and wore a rakish hat in this period... but he is often described as wearing a very worn, dirty blue suit, so he clearly only had one suit of clothes he could wear in society, such as his was.

Source: Kershaw, Ian. “Hitler: Hubris”, Penguin, 2010

Re. cafes, instead of Hitler, there's a review of A Rich Brew: How Cafés Created Modern Jewish Culture in a recent issue of the New Yorker that makes the point that even the grandest cafes were accessible to nearly anyone:

The Israeli Nobel laureate Shmuel Agnon wrote of his first experience as a kid from a town in Galicia visiting a big-city café in Lviv: “Gilded chandeliers suspended from the ceiling and lamps shining from every single wall and electric lights turned on in the daytime and marble tables gleaming, and people of stately mien wearing distinguished clothes sitting on plush chairs, reading big newspapers. And above them, waiters dressed like dignitaries.” All that for the price of a coffee.

...

The rule, still in place in much of Europe, was that you need buy only a cup of coffee to occupy a seat indefinitely. Customer loyalty is the commercial principle here. Better to sell the same writer a hundred cups of coffee than to sell a hundred writers one cup of coffee, since the hundred-cup man is almost certain to return for the next hundred, and the hundred after that.

There is an article in a Vienna newspaper from 9th February 1920 (below the cartoon) about the Café Central, which changed owner that year.

It alludes to the fact that "debutants of all art forms give their bouquets of laurel to the waiter if they can't afford the price of a Mocha".

It goes on to say that you will find people from all walks of life in the Central: "the ones still trying to achieve something, the ones who already have achieved it, the ones who failed to achieve it, the discontented, the hopeful, the disappointed, the believers and the godless."

At the end of the article there is a direct allusion to the fact that many of the younger and less well off clientele was running a bar tab and they were worried the new owner would be somewhat stricter in enforcing their credit lines, but apparently there was no reason to be worried.

So, it seems that a history of attracting an eclectic mix of people, some of them not very well off, might have made the ambiance of the Café.