People that find human infants cuter than rabbit, dog, or cat infants isn't a direct contradiction of the hypothesis, as humans would be particularly likely to find human infants cute (just as dogs are particularly likely to be protective and nurturing to puppies).

The point is that animals with large litters are particularly likely to have cute infants other things (like degree of genetic closeness) equal, and that large litter animals would be sufficiently cute to overcome the fact that we're not related. Of course, domestic puppies and kittens have an advantage over wild animals, as much selection was based on human popularity.

Thus, the question is whether you find say Infant Elephants as cute as infant (wild) rabbits or Wolf Puppies.