I'm going to be rude to my girlfriend's mother
Do you have anything against her?
She fails to do things for me whenever I visit her.
Assuming she hasn't done you any harm, it's expected that you are at least polite with your partner's parents. You shouldn't expect them to cater to your every need as they may be busy with their own things.
Sometimes my girlfriend can annoy me real good too.
You should be the one offering to help them. Showing respect to elders is really important. If you don't think she is a fit maybe you should reassess your relationship.