Most people think that because dogs are carnivores, they can't eat anything sweet. However, this isn't necessarily true - while their diet should be mostly protein and fat, small amounts of sugar aren't going to hurt them. In fact, some dogs actually enjoy the taste of jelly!
That said, there are a few things to keep in mind if you're considering feeding your dog jelly. First of all, make sure that the jelly you're using doesn't contain xylitol - this is a sugar alcohol that's safe for humans but toxic to dogs. Secondly, don't give your dog too much jelly at once - just like with any other food, it's best to start with a small amount and see how they react before giving them more.
So long as you're careful about these two things, feeding your dog jelly shouldn't be a problem. Just remember that it shouldn't make up a large part of their diet - after all, even though it's not harmful in small quantities, it's still not particularly nutritious!