Not nearly as important as some folks would like to believe. Justice Thomas is on record saying that they could do away with 'oral arguments' completely and lose little, if anything of value. This is one reason he so rarely participates in them. As far as he's concerned, all the meat is in the briefs, and the orals are nothing but theater.
On the other hand, I do think it would be valid to ask if any drugs she might be on could affect/cloud her judgement. My late wife had cancer, and went through two rounds of chemotherapy. She often complained about 'chemo fog', which was when the drugs she was on would make her very forgetful and easily confused.
Look up ‘supreme court mental decrepitude’ or ‘supreme court senility’. By historical standards RBG doesn’t rate an inquiry.