A kitten may be considered ugly because it is injured, sick or has a disfiguring medical condition, but this is very rare. While beauty or cuteness is in the eye of the beholder, baby kittens are generally considered cute because they possess traits that resemble the features of human babies.
Human beings have a preference for certain attractive features, regardless of whether these features are on a human face or on a baby animal's. Viewing pictures of young animals inspires much the same reaction as viewing pictures as babies, as shown in 2012 Swiss study entitled "Sweet Puppies and Cute Babies: Perceptual Adaptation to Babyfacedness Transfers across Species."
The scientist Konrad Lorenz termed these attractive features "kinderschema." They include traits such as a rounded head, large eyes and large forehead. Healthy kittens generally have these features, which inspires humans to find them cute. However, kittens that lack these features may be considered ugly. For example, a kitten that is injured and has facial deformities that affect its eyes may be considered less attractive, because eyes are an important kinderschema cue.