It wouldn't bother me so long as it didn't bother my child. If my child was upset by it and the other child was continuing to do it after my child said stop (and the other mom wasn't making a move to redirect her child), then I'd intervene and tell the other child to stop and explain that my child doesn't want to be touched in this way.
I have actually had to intervene a bit with my younger dd who is 5. She doesn't run up and kiss strangers (though I can't say that I'm absolutely sure it could never happen), but a couple weeks ago one of her friends who she often plays with at the park came and told me that dd was kissing her and she didn't like it.
So I told dd that you don't kiss people who don't want to be kissed. And it hasn't happened again. There was another occasion several months ago when she was trying to hold hands with and put her arm around a child who didn't want to be touched, and I just intervened and told her this little girl doesn't want to be touched. And dd stopped.
Shortly before that happened, dd had been playing with two other little girls and they were all holding hands and putting their arms around each other as they walked around ... then those little girls left and the new little girl came, and I think my dd was just trying to continue the game she had been enjoying so much.
Everyone has a different space requirement, and my little one, well, she doesn't need a whole lot of personal space and really loves to be in close physical contact with others. There's nothing wrong with that, but she just needs me to help her learn to respect the fact that not everyone wants to be touched.
I can understand that people who need a lot of space might not understand why someone else wants to be up-close and personal, and might even think there's something wrong with a child who's wired that way. By the same token, small children who like being up-close and personal don't automatically understand why some people don't like being touched.
Of course, the onus is on those of us who have sensory seeking children, to teach our children to respect everyone else's space, and not on the parents of the non sensory seeking children whose children are just minding their own business and not bothering anyone. But, still, a little compassion can go a long way!