In the most of literal senses, I would say that Williams did not make an error in having Amanda state that Laura is "crippled." Given the time in which Williams is writing, the term was used quite often. This doesn't make it right or appropriate, but by those social order's standards, it was accepted. This is something that we see quite often in that the level of verbal and social sensitivity in the modern setting is more heightened than it was in the past. It becomes a challenge to apply these standards to writers of the past. Certainly individuals have all the rights to be able to say that Williams should have used more sensitivity than he showed. (I still say that he gets a pass because he showed more sensitivity than most to the weaknesses of human beings.) Yet, in the time in which he is writing, to have Amanda refer to Laura as "crippled" does make sense. It's difficult to see what Amanda would have said about Laura in the modern setting because if we changed this element, the entire emotional timbre of the play changes. This is what makes Williams' work so profound. Change one element and it leads to other changes. Call it the literary "butterfly effect."
In a more symbolic sense, I would say that Williams did exactly the right thing in having Amanda say Laura is "crippled." For someone like Amanda, they would see Laura as somehow deficient. Amanda is gregarious, willing to construct her sense of self in a public manner that reconfigures her past in a nostalgic light. She is the type of individual that is not going to be at her best in trying to work with someone as Laura. She is not going to be emotionally sensitive enough to be able to see her as unique in her own light. Laura is nowhere near like Amanda. It fits her characterization to see Amanda as labeling her own daughter as "crippled" in her state of being that is not like her mother's. We can debate the use of language, but I think that given the point the Williams might have been trying to make about Amanda's character, he got it right.