I think that if many people stopped talking negatively about others, they wouldn't have anything to talk about at all.
And that's pretty sad, because it's such a habit they don't even know they're doing it.
But until your MIL herself asks, "Do you think I'm too negative about people?" there isn't much you can say or do. If you tell her how you feel before then, she probably won't understand, she'll take offense, and the problem will escalate. Probably.
I tend to keep my mouth shut and my face neutral, as if I am listening out of courtesy alone and do not wish to contribute to the conversation. The reason I do that is because I am listening out of courtesy alone and do not wish to contribute to the conversation. If it's possible to change the subject, I try to do it. If I can say, "Excuse me, I need to turn down the heat under the peas," I'll do it.
Here's the harder part: you have to be civil (and gracious, if possible) without bitterness. If you stew about this, you're hurting yourself. Change your focus (off her) as soon as you can. Mentally, throw her comments out with the trash as soon as you can. Even better, find a way to laugh it off when she has gone home.