Yes it's good and okay to drink lemon water at night before bed.
I drink lemon water at night before bed and I find that when I do drink lemon water before bed it helps me sleep better at night.
The lemon water is also good to help cleanse your body of toxins and drinking lemon water daily and or nightly helps to keep your body cleansed of toxins and helps you improve your health.
Lemon water is also good for the kidneys so it can possibly help prevent kidney disease.
Lemon water also contains Vitamin C which can boost your bodies immune system and help fight off infections and viruses and may even prevent the flu and cold virus and possibly cancer.