We have quite a lot of them.They're not generally called American themed restaurants (they're called "family restaurants" here), but the food is largely of that sort. One of them is actually quite tasty (and open 24 hours, so my friends and I sometimes wind up going there at 3AM... :'D), but the rest are all crap in terms of actual food, and only worth going to for puddings.
And we don't have a Dennys here, but I've been in other places in Japan, and yeah, it's basically that sort of thing. Not exactly very exciting. :'D