No, it’s not whataboutism. The original comment made a single argument: regressive taxes are bad. I provided a counter example: the cigarette tax is an example of a regressive tax that is good. This invalidates their argument. That doesn’t mean their position on tariffs is wrong, but they’ve provided an insufficient argument to support their viewpoint. There’s also an implicit corollary that they don’t fully understand tariffs if this is their position.

Whataboutism would be something like someone from the US arguing that China’s treatment of Uyghurs is bad, and someone from China countering with “well, what about America’s treatment of Native Americans?” The Native American argument isn’t a counter example of the Uyghur argument. Both positions can be true. It’s unrelated. That’s not the case here. You can’t be anti-tariff purely because it’s a regressive tax and also be pro-cigarette tax.