Was Carrie Ingalls on Little House supposed to be retarded?

We concluded that she was “slow”.

How many times is this question going to be asked on DL? You're dumber than Carrie Ingalls herself, OP.

No, I don't think Carrie was retarded, or even meant to be 'slow'. They cast two very young twins that ended up being unable to act, and they were so well liked by Landon and everyone else, so they didn't want to recast and Carrie became a total non-entity.

Nothing wrong with Carrie, except the sugar diabetes

As I was saying, nothing wrong with Carrie except sugar diabetes and a lopsided noggin.

She managed Mary, too, with some help from Grace.

In the books, once Mary goes blind, Carrie becomes Laura's companion. They go to school together and do chores, etc while Mary mostly sits in her chair by the stove, then goes away to college.

But yeah, the twins who they cast when they were toddlers (daughters of an actor friend of Michael Landon's) couldn't act, so they basically were treated as background.