Does Grandma do the pick-up every day? Or just once in a while?
If it's every day, I think you need to chat with mom. I presume she is the one paying the bill, and SHE is your client, not Grandma. Let her know you are feeling like G'ma is not happy with your performance, but that you do not see any major concerns. Ask her if she is happy so far.
If Grandma only picks up once in a while, then I think it's best to let grumpy dogs be grumpy. LoL
Do your best to smile and nod. Answer her questions, texts etc. politely, but direct and to the point. "Yes, he napped from 1-3 today, our usual nap time."