I'm going to ask my mom's best friend not to visit her anymore
Are you allowed to? Why would you not like her to visit your mom?
My mom's best friend is stupid and never does anything right
It is not your business who your mom chooses as her friends. You need to stay out of your mom's social life and let her choose who to be friendly with. You choose your friends and let her choose her friends.
I'm okay with being a mommy's boy.
It's great you really love your mom and want the best for her, but don't you think the best would be for her to keep her best friend? Her happiness matters a lot, so why don't you try hanging out with the two of them? You could some to like her.