Billy's mother is sort of like his wife, Valencia—Freud would have a field day.
She is incredibly insensitive and barely notices what's going on with Billy at any given time. When Billy is staying at the veteran's hospital after his mental breakdown, he cannot bear to talk to her because he feels guilty that, after all the trouble she went through to give birth to him, he doesn't even like life. Instead of trying to talk to Billy or to understand his troubles, she carries on a conversation with the man in the next bed. She discusses really personal things: Billy's business future, his upcoming marriage to Valencia, and all the day-to-day stuff in his life that he cannot handle any more.
The one time we see Billy's mother stop talking and gain some awareness is when she is ill with pneumonia in 1965, when Billy is 41. She looks around and asks, "How did I get so old?" (2.26.6). Her entire life has changed and she wasn't even paying attention.