not at all she is just one of the best if not the best musketeer companion. she has more promotions than most so she has more epics. personally i like billy the kid more but bonnie to me is not OP in any way and neither is any companion.
She isn't overpowered but she is definitely one of the best musketeer companions I think. I would rank her second behind Private Mills. On paper she would look like the best but Private Mills seems to critical more and survive better in battle. She is among the top 5 of all companions in my opinion.