I agree a lot of this is inspired by British culture (history, but also literature like Dickens) and by her Scotish sensibilities too probably, though she had several important characters who had the blood lineage and who were painted very positively - but most of them were either poor or nothing was made of their wealth (eg: Neville, the Weasleys etc.). Only Sirius and Harry were technically very rich, and sometimes Rowling was tempted to show this as a challenge/danger (and source of guilt) to Harry. He had to learn to use his wealth the right way, not to show off or embarass his poorer friends with over the top generosity etc.

Her harshest critic of the wealthy was of course the Malfoys. They treat their "staff" terribly, show off their wealth all the time, use it as a source of power for bribes etc. and they sneer at the poor, even the pure blood ones like the Weasleys who were to them a disgrace to the pure blood.

Let's say her own background (and even her poverty at the time) showed a bit in the way she depicted the mediocre middle-class wannabe bourgeois like the Dursleys and the insanely rich like the Malfoys, offering the poor, struggling Weasleys as her central positive family.

I'm not against it mind you, I found her satirical elements very amusing, but it was a bit black and white... maybe.