Should I insist my Mom to drink more water, limit her snacks, and insist she eat nutritious food?

I'd need to have more info about your mom's mental and physical condition. Like others have said, does she have diabetes? Dementia? A lot of health problems? Is she on a lot of meds? Is she depressed? All of those things can influence appetite. It sounds like she gets out to get the vending machine capps. How is she doing that? Give us more info about her situation and you'll get more good answers.

Do you sit down for meals? If so, make them nutritious and serve water to everyone at the table. If she likes the cappuccino how about suggesting you make at least one of them at home and make it with coffee and Boost or Ensure --at least then you will get some nutrients into her diet.

Elders lose their taste for most things and meals can sometimes be overwhelming. Try offering several small meals instead, 1/2 sandwiches, small cup od soup instead of a bowl, applesauce cut up fruits and veggies, cheese cubes, etc, readily available. Freeze some yogurt cups, replace candy with dried fruit, nutrition bars, etc.

Wow, 3 different answers and I agree with all of them. It really depends on her health, both mentally and physically.
Has she been checked for Diabetes? You say she is with it AND confused? If she is "with it" it's her choice. If she is confused, and especially if diabetic, step in.

I agree pretty much with what other people wrote. In your shoes I would make the suggestion to eat better. I would make the food available. However, I wouldn't force her to do anything she didn't want. 84 is a bit young to have such a deplorable diet, so I hope she will listen to your suggestion.

No. Make good food available. Let her choose.

At 84 I hope to eat what I like and crave. Unless I am a diabetic by then.
I would not stress too much over nutrition. Encourage her to eat, make things available, but let it be.

You should.