Cat Boomers are those who have gone though the "Catification procedure" (as Jill calls it), which was originally developed as a method to prevent Nanomachine Rejection in fetuses found to be at risk. Stella is the only shown Cat Boomer.
Jill explains that the name "Cat Boomers" originally referred to the parents who had their children undergo the process for the look of it, regardless of if the child was actually at risk of Nanomachine Rejection. Eventually people began calling those who went though the process Cat Boomers instead of their parents.
She also explains that the cat "ears" don't actually serve a purpose, and are just nubs of skin, cartilage and muscle, saying that it's lucky that the cat ears are the only major mutation due to the process, as other, less invasive procedures have done much worse.