Is there any truth to Winston's description of the capitalists in frock coats and top hats who ran London before the revolution?

The figure of the rich capitalist in a top hat is used by the Party as a propaganda figure. This is to remind people how bad things used to be in the old days before the establishment of Ingsoc. As with all caricatures, there is an element of truth to this. Indeed, the image of the rich, top-hatted capitalist exploiter wouldn't be anywhere near as effective if it didn't have some semblance of veracity.
The system of capitalism is undoubtedly exploitative. One doesn't have to be a die-hard Marxist to see that. At the same time, however, there is no subtlety or nuance in the propagandistic message proclaimed by the Party. There is no place for different shades of meaning in such a totalitarian society. Everything is either black or white. The previous capitalistic system, according to the Party, wasn't just wrong; it wasn't simply misguided; it was downright evil. And the figure of the fat, wealthy capitalist getting rich off the poor and underprivileged is a simple, straightforward image for the people to grasp, one that will divert their discontent from the present to the past. The message of this caricature is clear: if you think things are bad now (and if you do think that, then keep it to yourself because Big Brother is watching you), then just think what it was like in the old days when a bunch of corrupt, bloated plutocrats were in charge.