I understand not wanting to get to parties alone. I also understand having a close relationship with your mom. I don’t understand why you think you have to go to costume parties with your mom. You could go with friends or maybe by yourself.
Don’t you have friends? Or better yet you could just not wear a costume that is intended for couples. Maybe next time you could try a vampire or mummy. There are so many other options that I would try before asking my mom to the peanut butter to my jelly.