If you say she's happy, then let her be - just let her know gently that the constant messaging is difficult. My dad used to be like that - loved him to bits, but he used to phone at 8am when I was just leaving the house, and so on. Your mum maybe thinks you need the contact as you are away from home.
Your mum sounds like me - doesn't drink, and is very happy with her own company. Not everyone wants to be out gadding all the time. Not everyone wants to hang out. My greatest pleasure - when I'm on my own, when OH is away working - is to come in and know it's just me, everyone else can go away and leave me to my own company. I'm not lonely, I love being on my own. If your mum's happy, it sounds like she feels the same, but she thinks that YOU need the contact.
Get an answerphone, say gently that you didn't answer her call because you were out, don't always respond immediately to the texts. People are far too easy to contact these days and I hate giving immediate replies, I'm not there for their convenience....