Can I say my girlfriend's best friend is not hot to other people?
Are you serious? Why would you say that?
Because she is my closest competitor
You don't have to be mean to her to make your girlfriend feel better. It's not going to help and will make her feel bad about herself.
I can't help it. I want to say it so I'm saying it.
It's not nice to be mean like that. Beauty is in the eye of the beholder, so it's a bit subjective. You might think she's not hot, but guys might think she is, so maybe find alternative ways to close the competition that are not as rude.