I'm going to put my mom on a diet.
Why do you think your mom needs to be on a diet?
I would like her to lose weight, she's gained some weight recently.
It's understandable that you'd be concerned about your mother's weight but you can't just demand that she go on a diet. Let her lose the weight in her own time.
She gained 10kg in 2 months, it's very fast and she doesn't take care of herself. She's supposed to be an adult.
It is rude to demand people lose weight. It could be a really sensitive topic for her so if it is causing health issues you should gently let her know your concern.