what is important about nobody playing cards with mcmurphy to plot development?

I think this is chapter 11 that you are referring to. McMurphy wants to watch the world series; “After the patients wouldn't vote he got mad and skinned them so bad at cards”. I think this shows us how these patients react to McMurphy. McMurphy is a smart guy and easily manipulates them. The patients simply react to him (like not playing cards) but McMurphy stays in control.