The marginal utility of income is not measurable, whereas the percentage of income that gets paid as tax is. That's why "regressive" and "progressive" taxation is defined in terms of the measurable thing, not the unmeasurable thing. What you're saying is simply that a progressive tax system is better because it attempts to take into account the marginal utility of income.
And then everyone starts arguing about how progressive the system should be to do that properly. :-)
And then everyone starts arguing about how progressive the system should be to do that properly. :-)