Do you think it would be okay if I replace one of the players in my D&D group with someone else?
Why do you want to replace one of the players?
I believe him to be an average d&d player at best. He keeps making simple mistakes.
I don't think you should replace one of the people in your D&D group with someone else, because keeping him from playing would be rude. You should try to help him become a better player, not just kick him out.
But he doesn't know anything about the game.
He doesn't know anything now, but if you give him some advice then he might become really good at it. It's nice to give other people a helping hand if you have the ability to do so.