Accepting the premise, this outsider's view of the US is that there seems to be an increasing reluctance to fund "public" goods (e.g. infrastructure, population healthcare, etc) of which public education is one such service. Is this decreasing investment an actual thing, and could it (in aggregate) cause an overall drop in achievement?

US spend per student per year is $16,000. Fairly high investment on the international playing field, especially when adjusted for the poor results.

The view that the education institutions are bloated and inefficient is a fairly mainstream (because some things like low pay for the actual teachers in the classrooms is quite publicly apparent). I'd hazard to say that there's some truth to it, after taking in the first point into account.

Tl;Dr money is part of the problem on some levels, but it's not primarily a money problem.