I think there are bigger issues here and that your daughter playing basketball is not the problem. Is your ex very competitive? Why is she taking it so personally that your daughter is a new player learning the game and not as experienced as the other players? Did your ex play basketball? Is she insecure and feeling threatened that your daughter isn't the best player on the court?

If you can calmly talk to your ex (without either of you dragging in other unrelated emotional baggage), I suggest explaining that your daughter is having fun and likes learning from the coach and her teammates. She doesn't need to be the MVP to enjoy herself. You might also reinforce this with your daughter--that you're glad she's having fun and she should try not to let her mom spoil her fun