Nah, mirrors can lie.

(Or at least let you delude yourself.)
I don't think Monique needs the website to know if she's gaining or losing weight, but is more interested in finding out if the foods she's eating to gain weight are giving her a nutritionally balanced diet.
It's one of those interesting oxymorons of obesity (not that Monique is aiming for obesity, just that is probably the group of people that site is designed for) that people who are obese often have nutrient deficiencies in spite of such excessive calorie consumption, because what they are eating has a lot of fats and sugars, but not much in vitamins and minerals, and sometimes not even balanced proteins.
For another type of diet, those who follow vegetarian or vegan diets, such a site would also be very helpful to inform them if they are getting adequate fats and complete proteins in their diet. Getting complete protein is the biggest challenge for someone who eats a vegetarian or vegan diet, because you need to mix and match your vegetables properly. It can be done, one just needs to check it's being done right, especially if they are just starting out with such a diet and are learning how to be healthy on it.