That bend is intentional. On another bike forum a bunch of us compared our late 80s mixtes (from just before bike companies stopped making them) and a lot of them had that bent stay. I think it's a little ugly and the earlier ones are much prettier (they have a lug further up that sets the stay out far enough to clear the cassette), but it doesn't seem that it makes the frame structurally unsound.

One of my two Nishikis is like that. I confess that it's the one I never built up or rode, because once I noticed the bent stay and some cosmetic scratches on the frame I wasn't very interested in it. But I don't think it makes the bike unrideable.