I don't see why not. In raw feeding we advise people to not feed more than 15% of the diet unbalanced as far as calcium, taurine, and etc. which comes out to one day's worth of meals. Some kitties have problems with fish, something about it increasing UTI's. Some kitties become "fish heads" and won't eat anything else. But a simple treat? Nah, go ahead. Actually, if you could incorporate feeding a sardine a few times a week that's great for omega oils. I get the water-packed canned sardines.
I agree with peaches08: as long as it doesn't have any cooked bones and as a treat, I think it's okay. Ritz was a fish head before I transitioned her to raw and has a problem with UTIs so I avoid fish; otherwise, I'd give her some as a treat (she doesn't like sardines).