One has to separate out two elements of this - first is 'voter' and second is the difference between 'knowledge' and 'interest'.
American citizens have all kinds of information shoved in their face, but only about a third of them vote. Plenty of Americans are registered to vote, but that doesn't mean they actually show up at the polls. So your question limits the inquiry to those that actually vote.
Plenty of American voters follow a herd instinct - they grew up in some rural county and vote social conservative, or they grew up in some urban manufacturing center and vote 'liberal'. They may know all kinds of stuff, but that may or may not matter on voting day.
'Informed' voting requires several things: basic math skills, economic literacy, some sense of the lessons of history, and some actual understanding of their candidates. The math is need to 'balance the checkbook', so to speak - a lot of people simply cannot grasp what $17 trillion in national debt means. Economic literacy is similar - lots of people think voting for a particular candidate leads to a 'free lunch'. This is not limited to 'liberals' by any means - people that harp on immigration and border security don't make the connection between immigrant farm workers and the price of food. Neocons were all into spreading democracy in the Middle East, when that blew up spectacularly the US retreated into it's traditional isolationist mindset. Now we may be back in 1940, when the US didn't want to get involved in European wars.
Time and time again people return to office people that they wouldn't re-elect if they knew their actual views - for a long time people in conservative rural districts would send 'liberal' congresspersons to represent them. For various reasons Southerners would send Democrats to Congress, even though the political doctrines of these 'Democrats' were Republican in everything but name. This was a vestige of the Civil War and the Republican Lincoln administration - people have forgotten that now.