My MIL used to show up, maybe once a year, with a car literally full of horrible plastic crap. I... got over it. I really hated it, but I eventually concluded it was not a thing to fight over.
Here's my thinking, in several parts, with anecdotes (sorry for length):
Sometimes it helps to relax your standards a little. Or reconsider them. Non-plastic toys are not necessarily good, and plastic ones are not necessarily bad. Over the years, various relatives have given us push/ride on cars for small children. One was an adorable wooden truck with axles placed towards the center, so that when DS stepped on the end, the whole truck flipped upwards and smacked him in the face. I moved it to the basement, and refuse to pass it on to other children. Two of the cars were plastic Little Tykes numbers, which caused no safety issues. One of them had a variety of activities that made awful electronic noises. I removed the batteries one night when the kids were in bed. Both provided hours of entertainment here, and have been handed down to other families.
Your aesthetic judgment may not mean a lot to your children. "Ugly" is in the eye of the beholder. Let that one go.
Batteries come out. Let the kids have the full-on immersive electronic singy toy experience while Grandma is around, and then, after bedtime, get out a screwdriver, and yank the power. Consider putting batteries back in for special occasions, like the third blizzard to hit you in quick succession, or some day when you just need kids to give you space. I hated the Fisher Price Activity Table when MIL brought it over, but it got me a ton of uninterrupted cooking time.
Gifts are sometimes about relationships. Your MIL is giving gifts, probably, with some hope that the gifts will strengthen her connection with the children. If you don't like her taste, supply her with a detailed list of things you really want the children to have, that you think they would enjoy using with her, that are in her price-range. It usually goes over much better to say that you know your child would like (bath toys, a plastic slide for the backyard, a hula hoop, some spatulas and wooden spoons to play with) than it does to veto things she's brought to your house.
If you must, you can claim it's about you. And never do this in front of guests. Call a week later. "Oh Mom, DD *loved* the Fisher Price record player, but I got such bad headaches I had to put it away. Can we keep it at your house so she can still play with it?"
As your kids get older, they will develop opinions of their own about what they want, and you can use that information to help your MIL pick things you approve of. When you can talk to her as though the two of you are managing the children's presents together, you have a fair bit of control, but it takes some tact to get to that point.
(MIL died three years ago, and the giant plastic Imaginext DragonWorld Castle that makes the thunderstorm noises and has little canons that shoot pieces you can choke on is still in our living room. The kids love it, and remember their grandmother every time they play with it. I wish I had been more gracious while I had the chance.)