My DD is 4 and at play school. There are a few kids there with very challenging behaviour and also a few with ADHD and speech issues. My daughter (who is normally bright, quite articulate, fairly confident) has taken to copying, particularly the dominant kids with problems. If we see one of the kids who cannot speak very well in the park she will run around copying everything they do and mimicking their speech. I find it very embarrassing, and also infuriating.
I even noticed she did it when we went to the park with one of her old friends the other day, someone she is very much on par with. She followed the other little girl almost step for step, copying what she did exactly, even climbing things she could not climb without really looking because she was so closely trying to follow her friend. Even when her friend swung on the monkey bars and shouted 'I can do this!' my daughter, who had not heard what she said, swung on them too and shouted 'ba ba baaa ba' to mimic the sound of her speech.
AIBU to tell her to stop.