I don't know. I think that you really can't force someone to be excited, and getting annoyed at someone for not being effusive enough is really silly. I'm not a wedding person either, and a lot of people aren't. So, if the grandmother is really only causing this strife by, say, not talking about the wedding endlessly, the MOG is being a little ridiculous.
However, there's this *tone* to the letter and makes me think that's not all it is. I get the feeling if it were the MOG writing in, she might mention that Gma had made little PA or disparaging comments about the couple's living arrangements. And that's why she's upset.
I could see an over-the-top mother getting annoyed at her mom for not being over the moon enough. But in this case, I get the feeling grandma might not be totally innocent here. I feel like she is likely communicating her 'nonexcitement' in various ways. If it was just that Grandma wasn't really a wedding person, or had already seen some of her grandkids married and was burnt out, I doubt she'd have put in the little dig about the couple having been sexually involved for years.