I think I'd easily be saying, "well, I'm glad they're getting married, etc., but I don't understand what all the hooplah is about! They've been living as IF they were married, and we've been treating them as if they were married, so why all the money, and the party, etc.? It's not like this is something new and *exciting*, even if it is a good thing!"
To me, it does not automatically follow that she disapproves of the existing relationship, or even of them living together. But to expect, and even demand (as her daughter is doing), that people be excited about it it a bit much. Yeah, yeah, it's nice.