In my opinion, attempts to work out which culture or people is being stereotyped in fiction reveals more about the commenter than it does the author. In this case, it is probably a product of the annoying tendency to take Rowling's recent objectionable position on trans people and project that back in the past as proof that she has always been a bigot. Considering that Rowling has lived in Scotland for nearly thirty years, it would be exceptionally odd if she "meant" for the goblins to be stereotypes of Scots.

But I digress: in the eighteenth century there was a perception among the English that the Scots were, in the words of Linda Colley, "poor and pushy relations, unwilling to pay their full share of taxation, yet constantly demanding access to English resources in terms of trades and jobs." Following the Act of Union of 1707, the English taxation regime was extended to Scotland, which especially following the victory in the War of the Spanish Succession, included new duties on such commodities as beer, salt, linen, soap, and malt. Used to a relaxed taxation regime, which was not even sufficient to cover the costs of the Scottish civil government and administration, the Scots were aghast at these new levels of taxation, which was seen by many as an attempt by the English to pay down their National Debt. For their part, ministers in London were appalled at the scale of smuggling and customs evasion. An effort to extend the malt tax to Scotland in 1713 was so controversial that it could not be properly enforced, while a motion to dissolve the Union put forward in the House of Lords by the Earl of Findlater was only narrowly defeated by four votes.

This all fed into the 1715 Jacobite Rising led by the Earl of Mar, previously an advocate for Union with England, who rallied a force of 10,000 men in an effort to install the Old Pretender as King. Once the Rising was defeated, it was clear that something had to be done to ameliorate the impact of Union on Scotland. This did not proceed quickly: Sir Robert Walpole's government against extended the Malt Tax to Scotland in 1725, which led to rioting in Stirling, Dundee, Ayr, Elgin, Paisley, and Glasgow. Archibald Campbell, Earl of Islay (later third Duke of Argyll), was appointed to manage the situation, and reported on the almost complete lack of civil government in Scotland since the dissolution of the Scottish Privy Council in 1708. Islay would become the dominant political figure in Scotland until his death in 1761, with the responsibility of delivering political stability in Scotland and the votes of Scottish MPs to Walpole's government in exchange for patronage and the authority to govern north of Gretna. However, since there were never enough civil or judicial jobs in Scotland to go around to Islay's clients, posts in the East India Company proved to be an attractive alternative.

Disproportionate numbers of Scots would ultimately serve in the East India Company: between 1774 and 1785, 47% of writers in the Bengal Presidency were Scots, as were 49% of the officer cadets and more than 50% of the assistant surgeon recruits. The attrition from disease was enormous, but equally enormous fortunes could be made in India. John Johnstone of Westerhall returned to Scotland in 1765 with a fortune of £300,000, with which he acquired three estates. William Hamilton arrived in India in 1711 as a surgeon and was showered with gifts when he cured the Mughal Emperor of venereal disease, including an elephant, diamond rings, 5,000 rifles, and a presentation set of surgical instruments in gold. John Malcolm, born in 1769 as one of sixteen children of a tenant farmer in Dumfriesshire, was in many ways the classic Scottish Imperial success story: with little in the way of family connections or wealth, he joined the Easter India Company at the age of twelve, rose to the rank of Lieutenant Colonel, served as an emissary to Persia on two diplomatic missions, and became governor of Bombay before returning home with a knighthood.

Such was the Scottish elite's investment in the East India Company, as well as other massively-profitable ventures such as Glasgow's key role in the tobacco trade, or slave-grown Caribbean sugar, that when the 1745 Jacobite Rising led by Bonnie Prince Charlie broke out, there were no Scottish Lords putting forward votes to dissolve the Union: instead, the Presbyterian establishment in Scotland was horrified that Highlanders had sought to overthrow the Protestant succession. It would not be for many more years that the Highlanders, thought of as "barbarous and lawless ruffians... a crew of ungrateful villains, savages, and traitors", would be seen as emblematic of Scottish culture. Instead, Presbyterian Scotland celebrated the victory at Culloden and supported Cumberland's reprisals in the Rising's aftermath, because it was now invested in the Union and Empire.

Nevertheless, Scottophobia remained an aspect of British politics for some time: not only did the disproportionate influence of Scots in Imperial trade evoke envy, it was also seen as deeply embarrassing that in 1745, a small force drawn from the mountainous northern periphery of Britain had marched as far south as Derby before withdrawing. John Wilkes, a radical English polemicist, relentlessly attacked King George III's favourite, and the first Scottish Prime Minister, John Stuart, Earl of Bute, for his nationality, portraying him as a corrupting influence over the King even long after his retirement in 1763. As late as 1776, Wilkes was accusing Bute of influencing the British Government's response to the deteriorating situation in the Thirteen Colonies, something that was enthusiastically picked up by American Patriots. In the 1780s, Henry Dundas, nicknamed "King Harry the Ninth" for his dominance of Scottish politics, had to warn against advancing too many Scots into Indian service because he feared that their already-dominant position might incite another backlash.

In summary, the stereotype of the "miserly Scot" developed as a result of the rough early years of the Union, where Scottish resistance to new taxation met English exasperation at the perception that they were refusing to pay their way. This was intensified by the disproportionately-powerful position Scotland assumed in Imperial trade as a result of efforts to ameliorate the worst impacts of the Union on Scotland and reconcile the Scottish elite to it. As for the goblins, I myself doubt that they were inspired by stereotypes of Scots or Jews: the goblins of Harry Potter were inspired by the "Gnomes of Zürich", which is an old nickname within the British left for Swiss bankers, and one that J.K. Rowling would be familiar with given her long association with the Labour Party.

Sources:

T.M. Devine, Scotland's Empire: The Origins of a Global Diaspora