i would avoid it personally, or use it with care. your fish will probably taste it, but unlike zucchini, spinach, or peas (which are some commonly fed veggies) fruits (yes, tomatoes are fruits!) contain a lot of sugars. when they are left in the aquarium too long, the sugars get into the water from the flesh and the juices and make bacteria just go crazy. using them can make your tank really cloudy as the sugar-loving bacteria population explodes trying to consume all that free-floating, easy access sugar. same with corn.
you can definitely try it out - it won't hurt the fish. goldfish go crazy for oranges, which are on par with tomatoes as far as acidity. just keep in mind that you'll have to get your water-change stuff together in case your nice clean tank turns into a bacterial orgy.