Why would Rick specifically say "one" for Maggie?

Native speaker’s answer
Rebecca
He uses "one" when referring to Maggie, as he's regarding the Simpsons as a specific group of people. Generally, you wouldn't use this term when talking about people, but here it's like he's talking about them as if they're a species of animals. It's similar to when people see a group of animals, like a flock of sheep or a litter of kittens. There are no names for the animals individually. So, when you want to specify one of the animals, you can use "one" with a descriptive word. Ex: Wow! A flock of sheep. Look at the really fluffy one! Ex: I want a kitten, mom! Can we get a spotty one?