I think you sound like a great mum! Yes it is a difficult situation and awkward but you are still able to appreciate that she gets on well with your daughter and that your daughter likes her. I understand you feel a bit sad but I reckon it's one day of the year and she will never be your daughter's mother and possibly never have children of her own.
You have an opportunity to be the big person here and let it happen. The best thing that can come of it is that it nurtures an already good relationship with your daughter. You want that to continue so that your daughter benefits from a positive experience with this new woman in her life.
Are you worried the ex-MIL is trying to diss you? Perhaps she was just thinking it would be a good way to show her son's new partner that she is accepted into the family? I don't know without knowing more about your family dynamic.
I think it will be ok because YOU ARE HER MUM and no one can take that place EVER, so a petty card won't make the blindest bit of difference.
I wish you all the very best sweetie and a Happy Mother's Day to you from me! xxx.