Can I contest a Living Trust that my mom supposedly disinherited me in while she was mentally incompetent?

You can contest anything. Normally you want to be careful of a no contest clause. However, if you are totally disinherited then that's not a concern. That is, with a no contest clause it says you risk whatever you are given in the document if you contest. However, if you are given nothing then no risk there. I encourage you to find an experienced trust litigation attorney. Best of luck to you. -John