If you select the cooked mushrooms it’d be the weight after cooking, if you select raw the weight is uncooked. Remember to add calories in from any fat you use when cooking. Honestly though, mushrooms are very low calorie so I wouldn’t be too concerned, other foods I’d be more careful.
Unless the package states otherwise, it doesn’t matter whether you weigh an item raw or cooked.
What does matter is weighing your food in a manner that’s consistent for you to avoid confusion, for some this is weighing all their foods raw and for others it’s weighing all their foods cooked.