Kiara is relatively normal.

1. Mufasa would have fathered all cubs in the pride from the moment he became the dominant male. 2. Children of relatives don't automatically end up as freakish mutants with arms growing out of their foreheads and whatnot. It just increases the risk that recessive traits such as leukemia and anemia will show up in the children, because both parents may be carriers. There's still a ~93% chance that recessive traits won't show up in the first inbred generation anyway. 3. Most animals are much less genetically diverse than humans, and their bad recessive traits tend to get bred out of the gene pool eventually. You can bet that if lions had diabetes (for example), all the diabetic lions would die for lack of medical care and there'd be no danger of it showing up just because Simba did the deed with Nala. 4. This sort of breeding is common in pack animals with only one male. Besides, the dominant male may be replaced every few years just to ensure some diversity. Once the males age they are no longer as able to resist challenges by younger males.

They are animals...

OK, these are LIONS that we are talking about here. Part of the ANIMAL KINGDOM. I don't know about you but my pet lions did not have a marriage ceremony before they decided to have cubs.

didn't have to be Mufasa..

ever consider that maybe Scar is Nala's father? Or, maybe there was another male lion in the pride before but he died or left. If it were real, Mufasa would probably have killed Nala at that point, but it's not real and maybe he was just nice.

Natural Selection

First of all, the risk of a genetically mutated cub would be slim if Mufasa indeed fathered all of the cubs in the pride. Through natural selection, all inheirited-diseased lions would die and healthy lions would be left. Second, not all children born of closely related parents have their heads on their butts. My tom cat impregnated his mother, and their kittens turned out 99.9 percent healthy. In fact, domestic cats and lions are both members of the genus [i] Felis [/i] so the child of closely related parents in Felidae souldn't be any different when it comes to mutantcy rates. Third, humans are in the Animal Kingdom, or Kingdom Animalia. The other Kingdoms are Eubacteria, Archeabacteria, Euglena, and Plantae. Anyone who has taken Life Science (or at least passed seventh grade) should know.

Animal genetics

I grew up in a small town. Not rural, not farmland, but small. My grandfather loved cats and had (literally) around 50 cats on our 5/8th of an acre. There were usually no more than three or so males and only one dominant male on the property at any one time. Cats would often give birth to kittens that were created from brother/sister, father/daughter, grandmother/grandson, etc pairings. Genetic defects in healthy animals, if they occur at all, take many, many generations to start showing up. In the 18 years I lived at my grandparents' house, we only saw maybe 5 or 6 deformed kittens.