There is no kitty equivalent to the Down's Syndrome in humans. There are congentital and infectious disorders in the feline in which their brain's fail to develop normally and the outcome is death or inability to walk or balance, for example. Many feline fetus's that have abnormalities miscarry before birth, or, if they go full term and are born, are usually abandoned by their mothers, because the feline mother can sense the abnormalities much better than their human counterparts. Some mother cats will even kill their abnormal offspring, and even though that may sound sad the mother cat knows instinctually that the abnormal offspring wound not live very long on it's own.
So if your cat is acting normally in every way, and , seems to have normal eyesight I probably would not worry too much right now. However, I would have her examined by a vet, (which you should do anytime you get a new pet), and have all the routine tests performed and any other tests deemed necessary by your vet just in case. You can also post a link with a photo of her on it for us to see.
Perhaps it's a normal variant or mutation. It's been years since I've been following cat breeds. I recently borrowed a CFA book from the library and was amazed at the new varieties out there. There are a few that have unusually round eyes, kind of cute but also a bit eerie. Hope the kitten is fine and you have a happy new member to your family.