Would it be OK to ask my mom not to be on her phone while we are watching a movie?
Well, are these important phone calls?
Not really, and it can be distracting to other people.
It's totally understandable to be annoyed at another person when they are on the phone while the rest of you are watching a movie. The bright screen is distracting and it is rude since it negatively affects how others enjoy the movie. As long as you are respectful in your request, it is perfectly acceptable to ask your mom to not be on her phone.
DAMN I was kinda planning to abuse her while asking.
You're probably joking, but if not, a polite request will be best. You're more likely to have it respected if you're not abusing her.