Can I tell my boyfriend's mother that her house is gross?
Why would you tell her that?
I don't like to talk to her, and if I have to talk to her then I'll be as direct as possible.
Even if you don't like your partner's parents, you should try to get along with them. Your boyfriend's parents are important to him and if you treat them badly he will feel hurt and may not trust you. You could be sabotaging your relationship by being rude to his mom.
It's fair to return the same amount of respect I get from her.
Do you love your boyfriend? Because if you do then you'll try remaining respectful to his mother and just not spending a lot of time around her-- being too direct could mess with your relationship.