This explains physically disabled treatment in china.
https://www.prisoncensorship.info/archive/etext/classics/lifeundermao/deaf1.pdf
https://en.m.wikipedia.org/wiki/Disability_in_China
This one is about the USSR but still relevant.
https://isreview.org/issue/103/disability-and-soviet-union-advances-and-retreats/index.html
but to answer your question communism is based on "From each according to his ability to each according to his need." so as long as the material conditions allow for it mentally disabled should be treated well.
https://www.scribd.com/document/256390020/Disability-Studies-a-Historical-Materialist-View
This explains physically disabled treatment in china. https://www.prisoncensorship.info/archive/etext/classics/lifeundermao/deaf1.pdf https://en.m.wikipedia.org/wiki/Disability_in_China
This one is about the USSR but still relevant. https://isreview.org/issue/103/disability-and-soviet-union-advances-and-retreats/index.html
but to answer your question communism is based on "From each according to his ability to each according to his need." so as long as the material conditions allow for it mentally disabled should be treated well. https://www.scribd.com/document/256390020/Disability-Studies-a-Historical-Materialist-View