Hello there , when I had chemo I would usually have an hours nap in afternoon .I do think your body needs the sleep to heal . Is your mum up and about when she is awake ? I think napping is fine as long as you don’t take to your bed for too long as there are other problems because of immobility. 
Does she enjoy going outside for fresh air ,it could be that she is feeling low too .

Best wishes and keep in touch x

She is very down which is understandable I think but she's also just exhausted - she's napping for half an hour before lunch, and then again around 4ish but she just wants to nap all day. I'm not sure how to support her - I've been asking what she needs or what she wants but she doesn't know herself.