Is it wrong to put mud in her food?
Do you really want to do that to her food?
Yes, she deserves it.
Just because you think they deserve it, doesn't give you the right to serve them food that is contaminated. Putting dirt in her food can make her very sick. You should leave her food alone.
What if she doesn't find out what I've done?
I'm pretty sure she will taste the mud in her food. You could get her seriously ill, please don't mess with other people's food like that. If you are upset with her, find a harmless way to confront her over it instead.