I don't want to go to my girlfriend's parent's house so I'm going to avoid a visit.
What's the reason for not wanting to visit them?
To be blunt, her mother hates me
It's natural to avoid people who hate us. But you should also try hard to get along with your partner's parents. Maybe if you visit more often, she'll grow to like you.
Very funny, I'm just going to text them that I won't be able to make it.
It's perfectly reasonable to avoid those who don't like you, but you should put in extra effort with your girlfriend's mom, as that is an important relationship. Keep trying to get to know her better and she will probably come around and learn to genuinely like you.