Not all camps and lodges in Rwanda have mosquito nets. It all depends on their individual location, e.g. the altitude, distance from open water and so on. Much of the accommodations are tented camps which are sealed tightly and zipped up, avoiding the need for mosquito netting.
In the lodges, while you enjoy your dinner, house-keeping will complete a turn-down service and spray the rooms with mosquito repellent.
Regardless of mosquito netting, you should always bring insect repellent with DEET, trousers/slacks, sports/long sleeved shirts and/or blouse and dress/skirt for ladies. Keeping your skin covered will reduce your risk of being bitten.