I hate my best mate's girlfriend
Why do you hate her? Has she done something bad?
She has lots of flaws and some stupid reasons for not liking my best mate.
I can see why you aren't very fond of her. It's okay to not like your friend's dating choices. Just try to be there for him in case things don't go well for him.
Why should I?
It's important for friends to support each other. Since she sounds like a not-great girlfriend, they may not stay together. It would be nice for you to be kind to him if a break up happens.