I actually went through something similar with my own DM! I came from a simple working class background, whereas my first DH was from what you'd perhaps class as a 'white collar' background. Due to this, my DH had a strong desire and ambition to provide for us, in the same way his DF provided for him and his family. My DH worked hard, and with time and effort, gained promotion in his job, which ultimately allowed us to buy our first detached home, after having lived in an old terraced place that we'd bought as wreck, and worked hard to do up.
At that point my DM who was my absolute world, (other than my DH and child) started getting nit picky and bitchy with me every time I saw her, which was incredibly unusual as we always got along like a house on fire. After a few months of this, I decided I couldn't bear her being like this with me any longer, so I tackled her about it. She started off by saying that I had become 'unapproachable' since moving to the new house. This was total bollocks, and it turned out, that it was actually my DS and BIL, who had been dripping poisonous remarks in Mum's ears, about how snooty we'd become, and that we were always buying bigger and better things, so were clearly getting above ourselves. I should say at this point, that my sister has ALWAYS suffered from the green eyed monster, and her DH seems to have the same traits. I asked Mum, in what way was I different from when I'd lived in the terraced house, and in the end, she was forced to admit that it wasn't actually me that had changed, but that she was frightened I might grow away from her, because I could now buy things that she and my DF, could never have afforded. I told her that NOTHING, would ever make me grow away from her, that I loved her dearly and wanted my parents in my life, wherever I was. We had a huge hug, and from then on, went back to our usual loving relationship. It was years later that she told me about my DS & BIL.
The reason I've told you this OP, is that perhaps your MIL thought you were lovely until she discovered your background, and then found herself feeling unable to compete with the wealth that you had whilst growing up. Perhaps she felt intimidated by it, and so decided to try and show that she didn't give a toss, by not bothering to dress up for your wedding etc.
In your shoes, I think I'd perhaps be inclined to have a quiet chat with her, tell her that you thought the two of you got on really well, until the time she came to your family home, and since then, you feel that she's gone off of you, and you don't understand why, but you miss the person she was, and so if it's because your parents are wealthier than she is, you'd just like to reassure her that you love her for herself, and not for what she can, or cannot, give you and your family. I'm hoping that if you're able to do this, it might just make her realise not only that you've noticed that she's unhappy about something, but also that you value her.