Oooh, I know what you mean!!
My blood still boils when I think about when DD was 3 y.o. She used to get, I admit, the world's cutest pouty face when she was on the verge of crying. So my MIL (!) one time thought it was a great idea to play it up when DD got the pouty face for I don't remember what reason. MIL was holding her on her knee and kept yelling in a sing song voice, "Awwww, don't cry, sweety, awwww!" all the while laughing and trying to get her to cry some more!
We were at a party and it was already loud and I was on the other side of the room, but I politely said, " Bertha, please stop, you're making her cry. " MIL didn't even hear me and went on with, "Awwwww! Look at
the pouty face!" and DD was, of course, just getting more upset, tears streaming down her face.
"Bertha, stop it, please"
"Awww, she's so cute!"
I strode across the room and got in MIL's face and loudly said, "Bertha, stop,
that's not cute!!" I took DD from her and went outside. I thought I was calming DD daughter down, but I was shaking I was so upset. I don't regret that for a minute, in fact that was kind of a break through for me and my relationship with MIL. My first "mother bear" moment! Sorry, Bertha, but you DON'T know what's best for MY child!!! But when we went back in I tried to smile at MIL and say, "sorry!" I tried to say something about PMS.
It had the desired effect. MIL backed off after that.
You totally did the right thing!