No, that is not okay. Fish should never be a major part of any cat's menu. Not only does it lack essential nutrients, but tuna in particular is dangerous in larger quantities because it is often contaminated by high levels of mercury. Mercury acts as a neurotoxin, even in small doses! It's fine to mix some canned tuna through a bowl of regular food as a treat sometimes, but do not make your cat's entire diet consist of only tuna or any other type of fish.