I want to tell my daughter to call her boyfriend 'Mushmouth'.
Why would you want your daughter to make fun of her boyfriend?
I don't like him and I think he doesn't deserve my daughter. He's an ugly asshole.
I know that you don't like your daughter's boyfriend, but telling her to call him names is impositional. Mind your own business and don't tell your daughter what to do.
They are just words. It's not a big deal.
It may not be a big deal to you, and to many people it won't be, but you never know how others will react to words. By imposing this demand on your daughter, you could not only harm their relationship, but cause your daughter to lose respect for you. None of these are worthwhile goals.