To those saying marriage is no big deal, it's just a legal device etc, it does have an impact on alot of things, otherwise divorcing wouldn't be such a ball ache. In this case it's being used for financial protection in later years. It does play into things like who is NOK if nobody else is specified - if you have a legal spouse they tend to be the default, at least in my experience.
In the case of my DF and not so DSM, together 40 years, married for 25, they have separated for various complicated reasons and dealing with various official bodies from HCPs down to council officials has been a blooming nightmare because of various interpretations of and attitudes to their married status. Both in their 80s, now with solicitors in the mixand honestly I could cry. No divorce on the cards yet, but God knows where it's going.
After they separated, it took me months to get through to my Dad that he had to change his will to make sure that she didn't automatically inherit everything (he has no assets, she owns a property). Also to make sure that if anything happened to him that she wouldn't be automatically given a say in his care or his funeral arrangements, because she would have taken great delight in making my life a misery in such circumstances. She's a tiny dot of a woman but she could start a fight in an empty room and if she said the sky was blue I'd need to see it with my own eyes.
At one point last year, when we were still on speaking terms and I was doing hospital visiting every other day because DF was too unwell, she spent an hour gleefully plotting his funeral and informed me that I would probably be a bit upset, so I could go to the funeral and have a couple if friends to support me, but as the wake would be at a "naice" hotel, we should only stay for an hour as they wouldn't want us lowering the tone, and there would be no alcohol. I stopped swigging meths from the bottle in a brown paper bag at that point and gave her a bit of mild side eye. Obviously that's a joke. Yes, she's unwell, but she "has capacity" so she can say and do what she likes apparently.
Sorry for the tangent, what I'm trying to get across is that marriage being just a formality of some sort is an extraordinarily simplistic view for both legal and cultural reasons, and it can shape and influence all sorts of things including emotional ties and practical/legal issues.
OP wants to mark the occasion from affectionate motives, but is being frozen out for reasons which seem clear cut in her mother's eyes, but not to her, given the build up, and it is rather cold. She is allowed to feel how she feels, and I feel for her.