Here's the thing. Two things, actually.
First, Bachmann's knowledge about John Wayne strikes me as precisely the sort of small town mythologizing that happens all over the place. After all, Waterloo, Iowa has no real claim to fame, right? Might as well glom onto some celebrity whose family slipped through town for a little while. Who can it hurt? The fact that Bachmann took this as accepted wisdom only shows her lack of research skills (or desire), as shown by nearly everything that comes out of her mouth. This is hardly her most grievous sin.
Here's the other thing - Gacy wasn't really "from" Waterloo, either. He was born in Chicago and did most of his evil deeds there, although he did get his start in the couple of years he spent in Waterloo. But he's no more "from" there than Charles Manson is "from" Charleston or I'm "from" Morgantown. You're "from" where you're born or, at least, where you grew up. If you're going to ding somebody for getting their hometown history wrong, you damn well better get it right.
Point is, this is no more than the partisan point-scoring bullshit that passes for political dialog these days. Knock it off! There's serious problems afoot. Not that Bachmann is the woman to solve them . . .