"The cats are cuter than the dogs" - if you and your listener talk about cats and dogs that are in front of you, or when talking about specific animals you have both seen somewhere, then using the is appropriate.
Same with singular, "cat" and "dog".
If you are talking about all cats and dogs in the world, so you don't have any particular dog and cat in mind, then you go with "a" for singular and nothing for plural.
So you would say:
"A cat is cuter than a dog."
"Cats are cuter than dogs."