As it happens, the voice actor who served in the offending episode as the voice of Chris Griffin’s love interest, Ellen, herself does have Down syndrome. I think this is the money quote from the NY Times article explaining this:
And when I watched on Channel 4, on “Extra,” and I saw Sarah Palin with her son Trig. I’m like, “I’m not Trig. This is my life.” I was making fun of Sarah Palin, but not her son.
There’s a lot in this piece that is humbling, at least to me. It changes the way in which I think of people with trisomy 21.