A similar thing has happened to me before - at that age kids know that that's not acceptable behaviour, DD is just over 2 and wouldn't snatch and scream over toys that aren't hers. We were in a play cafe type thing when DD was nearly two, and she got pushed off the steps of a little play house thing. I saw the other child's mother watch the whole thing, but DD git up and went to something else. The little girl followed, and snatched the ball DD had picked up. Other mum went back to her phone, DD went to look for something else. She then got pushed off the rocker she was on and had had enough, she started crying. The other mum said that's not nice, and went back to her phone!

So I went to DD and picked her up off the floor. I stood her up and hugged her, with the other little girl (maybe just under 3) just standing there. So I said to her it's not nice to push or snatch, she'd made DD very sad and we all need to try and share the toys and equipment nicely. She just stared then ran away. There's no way the mum couldn't have heard given how close she was, she just didn't care.

I'd probably have done similar in your situation given that your DD was being completely disrepected and bullied, and the mum didn't care. I'd have stood up for my DD if the other mum wanted to have a go, but as you didn't let it get to that your option is probably better than mine! And I agree that it shows your DD to be the bigger person, especially in a public place, and you were able to explain the situation to her after.