The 'tax' part only kicks in if you a) don't have insurance and b) decide not to buy insurance. So that makes this a bit trickier to answer as we have to make assumptions as to who will decide not to have health insurance.
But let's ignore that part.
If we treat it merely as a purchase one has to make, we could say it's "regressive" in the same way that buying a can of soda is "regressive". For someone that earns less money, more of their income percentage-wise has to go towards that purchase.
So, the less money you make, the more "regressive" this purchase will be.
That would be true down to the point where you meet the subsidy thresholds, which are between 100 and 400 percent of federal poverty levels. At that point, you could argue the purchase is now "progressive" in comparison.
To answer the specific questions literally:
Is the ACA a regressive tax?
No, it's not a tax. There is a penalty component, which is considered a tax by the SCOTUS.
Which income quintiles are most affected?
Hard to say with any certainty because the penalty is not tied to income, but whether or not a person chooses to avoid having a health care policy. Once we see who makes those decisions, we can then determine if they fit into particular income demographics.
It should be noted, however, that the penalty is tied to income above a certain threshold, so one would likely not consider the penalty, in and of itself, regressive.
Summary: One could argue that the concept of having to purchase insurance is regressive through a particular range of the income spectrum. The penalty for not purchasing insurance is not (again through a particular range of the income spectrum).