As a patient of GBM I can relate to your mother's indifference toward what she used to like, but I also feel sorry for you in your frustration to help. I wish I had suggestions for you to occupy her time, but I have the same problem. I'm 53, used to work full time as a self employed salesperson, took care of my elderly mother who lives with me, multiple pets, did all housework, laundry, shopping, yardwork, etc. Now my vision is impaired so I can't drive, can't work, etc. etc. But I also find I can't handle too much stimulation even though I have few deficits from the surgery and radiation. Conversations are hard (I've learned to text message instead of talk on the phone), groups of people make me agitated after awhile==even my family! I start to stutter and mix up words. I bought a camera hoping to pick up a new hobby--can't see well enough, not interested. I, too, longed for spring to work in the yard--I do when the weather is good (rare here in New England) but it's not as enjoyable as I'd hoped. Feels more like a chore than a hobby. Tried journaling--got bored. You get the picture. Luckily, I have finally mostly come to terms with my disability and THE NEW ME. I say "mostly" because there are many days when I'm bored. I remind myself that life was often boring before I got GBM. Anyway, could be your mother is immersed in her own thoughts. I've always been an insular person but am much more so now. And maybe she's still feeling guilty about not being the person she was before brain cancer but can't (or doesn't want to) admit it out loud. I've read your previous posts but forgot how old she is and what she did before GBM.
I didn't mean to write so much about myself, but since I don't know your mother I thought I'd chronicle how I feel and hope there are similarities you can latch onto that might help with her psyche. A way to pass the time will come when she accepts who she is and what she's capable of.
Good luck, Mary dx 9/2007