I so feel your pain. My MIL is the same way. She goes way over the top. We return 90% of what she buys every year. I don't say anything about the amount of things she buys though, because it is her joy. She is retired and does not really do anything with her time. She starts buying for Christmas in August and just goes crazy. But she is so happy with her dining room piled high with boxes and when the UPS man comes to drop off packages, I can't take that away from her. It is how she shows her love, as twisted as that sounds to us. She has been watching DS1 two days a week for a few months and every time she comes, she brings him at least one thing (usually a matchbox car and cookies). I am constantly trying to simplify our lives and home and choose not to have all those things at our house, but that is not something she can understand. We still have toys in their packages from DS1's first Christmas. I always thought we would open them someday when he ran low on toys. HA!
I just wish she was more thoughtful in her gift choices. We told her before we had kids that she should not buy the plastic battery powered toys she was buying for our nephew, because we did not want them in our house. The first two years she did pretty well but, this year was battery-apoloza. It was crazy. Luckily, she includes gift receipts now. It took us years to convince her to get them. Every year she buys me some sort of cheap yellow-gold jewelry. I don't wear jewelry -except two rings which are NOT yellow gold. She has never seen me wear any of this, but I get a new one every year.
After everything is open, and for weeks after, she asks us over and over if we liked all our presents and if everything was ok.
So I guess you have to decide if it is worth making a fuss over. For me, it's not. We do a VERY low-key Christmas, in part because I know MIL will go OTT, and I know that the values of meaningful and minimal gift-giving are what will stick with my boys since that is how we live our everyday lives.