@Asiama - she sounds dreadful. I think maybe you should go very low contact with your Dad too, and even consider blocking his number, or getting someone else to screen "his" messages.
The message she sent you is such a dig - no care about you at all, but reminding you that you didn't send her a card or anything either. Very unpleasant.
While I think the "nasty message mum would send" reply would be a good one, I don't think I'd even give them that satisfaction - I'd ignore it totally.
Have you heard of "grey rocking" people? It's when you react like a grey rock - you deflect everything, nothing affects your inner self, and you show no reactions to anything they might throw at you either. It's very effective when dealing with narcs and emotionally abusive twats, and their flying monkeys (look that up too if you haven' t heard the phrase yet).
Practise grey-rock techniques so that whatever your mother tries, you can just shrug it off and carry on - you'll even start to feel pity for her pathetic attempts to get to you after a while.
I know what you mean about feeling a sense of loss regarding your relationship with your mum - I had this too, I was never close to my mum, she didn't seem to know how. Not outright abusive but close to it - very self-centred, no empathy, everything had to be about her and her feelings. She's been dead for nearly 14 years now so doesn't cause me any further grief in that way though.
HOpe you did have a happy Mother's Day, despite your own mother's best attempts to spoil it for you!