Unless ordered otherwise, there is no reason that you cannot assist at your daughter's activities. Kid's sports are always in need of parent volunteers. If she really enjoys the activity, there is no reason for you not to enjoy being involved with her. If you do, make sure your court orders allow you to assist or attend even when it is not your custodial time.
I do expect that if her mother is doing this to aggravate you, your being willing to participate will stop her.
I do expect that if her mother is doing this to aggravate you, your being willing to participate will stop her.