Everyone has heard jokes about mother-in-law. There are hundreds.
What you have to remember is that almost everybody in the world has a mother in law. That also applies to your own mother in law!
The most important thing in a mother's life is the welfare of her own child (s). She was involved in childbirth delivery, from school to school, to comforting minor accidents and grief, from paying school fees to perhaps the first car. The list is endless and all these things were done to make sure her child was happy, safe and ready for the big world.
Then you came in. Whether you are a brain surgeon, the first female astronaut on the moon, a billionaire or a waitress, you are a stranger to your mother in law. The long-term care of her child just transferred over to a stranger, and this is not an easy thing to deal with.
And you? We all have a mother, and know that they have our best interest at heart.
See your partner's mother in the same light as yours; she's not just there to make a fuss, she really cares, and will do everything she can to help.
So what can you do to make things easier? Make a list of things she can help you with. This way you have control over how much responsibilities she has. If there are clear boundaries it helps everyone know their place.
Don't criticize everything she does; be thankful that she's there for you, but make sure she knows the boundaries of being helpful and being interfering.
There are some situations in which you would like more help and advice from your mother-in-law. A simple, honest, conversation with her goes a long way.