That depends entirely how you measure. If you measure by total income, then yes. If you measure by income after necessities for living are accounted for, then a very simple progressive tax that takes that into consideration might be considered the baseline, and a flat tax that doesn't take that into consideration could be considered regressive. On the other end of the spectrum, if you make people account for all subsidized government services used and count that as income, then a flat tax is progressive. Without an agreement on the baseline for measurement (which I believe is where a lot of people start disagreeing), you can't even necessarily agree on what is progressive and regressive.