How can I set boundaries with MIL?

My MIL is constantly criticizing the decisions my partner and I make with our two month old son. She is one of those people who worry about everything. So she basically never even leaves the house because she feels safest at home.

We got our baby a tummy time mat and put it on the floor in the living room. She immediately was like “ that’s so scary. that doesn’t look safe.” We told her it was fine and that’s where it belonged and continued to watch and play with baby on the mat. She was like “ baby tell them it’s too hard on the floor and you’re uncomfortable”. And the vibes in the room were just horrible because she seemed annoyed with us.

There have been many many instances like this since we’ve had our baby. She is always making comments about how we pretty much are doing something wrong and overall is just super judgmental about our decisions.

I’m tired of not speaking up. How can I get her to stop doing this? I don’t want to be confrontational and I just want to start saying something that’s straight to the point so that she can’t argue.

How can I set boundaries with MIL?