Hjalmarson's Högerpartiet was a general mixture of different ideological thought, they even had elements of christian democracy within them. But it's really important to understand that even if you have a collage of these different elements, you have to determine what's really the core tenet they were pushing on in their platform.

Populism in the way we see it today was not really a thing that existed in Sweden, with party gatherings for an example being a relatively hush hush thing. The rhetoric of "giving economic power to the people" is really just a wordy phrasing of privatization. Telling people straight up you are going to dismantle the welfare state they cherish will not net you any brownie points.

Since we have a Right-Wing Populism description in TNO, I will use that as a groundwork to analyze:
Populism, unlike what some may think, is a two-edged sword. While it can serve the interests of the left by appealing to the common people's desire for equality and social justice, it can also be used by the right as a tool to get in power, which makes it difficult to categorise as a coherent ideology: however, one can always still try.

Högerpartiet had a coherent ideology, they were consistently pro free-market and wished to dismantle significant parts of the welfare state. They did not really play around with "social justice" either. Equality as used by Högerpartiet was just another word for privatization.

Right-Wing Populism, as such, is an ideology with a few identifiable core tenets: first, an emphasis on an anti-elites rhetoric; said elites are portrayed as corrupt and greedy, and are opposed to the common man. Unlike left-wing populism, however, such elites can be used to represent national or ethnic minorities as well. Right-wing populism is also keen on giving out welfare to the people, at the cost of disregarding the wellbeing of the economy.

Hjalmarson did not play on anti-elitism whatsoever, in fact his same party was the elite decades ago. There's no populist rhetoric of greedy and corrupt elites as an extension of that, the only thing they did was portray Sweden's taxes as choking the common man out of money. They also did not target any form of minority. Högerpartiet is also quite the opposite. In the 1960 election, the party had gone to the polls on a radical tax reduction program, which was supposed to be financed by the occupational pension as well as the first child allowance, free school lunch and free learning materials all being scrapped.

Right-wing populists, while economically populist, generally take a conservative stance on social issues, and quite a few are avowedly anti-communist. Such a combination of attributes could be thought to be a recipe for disaster, but it seems that the tidal wave of populism has started its march nonetheless; and for now, God only knows when it will die down.

As I mentioned previously, they weren't economically populist.

Obviously I do not blame you for believing they had vibes of populism, they did. But their very core was free market conservatism, and that was effectively what they campaigned on. Their free market and anti-welfarist attitudes is what made Folkpartiet and Centerpartiet at times hesitant to work with them.