Even ignoring the disposal income question, it's regressive in utilitarian terms because money, like most commodities, has declining marginal utility, so taking 20% of $1,000,000 takes a lower percentage of the utility of that $1,000,000 than taking 20% of $100,000 does of the $100,000.

(A lot of analysis gets based on the misuse of $ as a flat measure of utility, which is deeply flawed.)

EDIT: more accurately, this subsumes and generalizes the disposable income issue, rather than ignoring it, since the concept of "necessities" that come out before income is "disposable" is really a simplification of the "low hanging fruit" of high-utility spending to a binary category, rather than a continuum.