I follow Weight Watchers. Everything has a point score allocated to it, and you get a certain amount of points to eat per day. Muesli is a lot of points in comparison with other cereals like Corn Flakes, All bran flakes.. even the sugary rubbish is less points than Muesli.
So no, just eating Muesli will not help you lose weight. You will not be providing your body with all the varied things it needs to function properly. That has to be the dumbest idea ever.
Want to lose weight? Stop eating chips, drinking fizzy drinks, stay away from processes carbs, especially at night. Eat more vegetables, drink more water. Eat more fruit, and always choose the low-fat or fat-free options. Reduce your meat intake. Deskin your chicken. Stay away from fried stuff, especially deep-fried or crumbed stuff. Don't eat anything that doesn't have nutritional information on it. Write down what you eat and drink and the associated kilojoule and saturated fat amounts. You'll soon notice what is a lot of kJs and fat for little "feel full" value. Eat a large breakfast, a medium lunch, and a small supper.
Drink more water. Do some exercise. Even just 15min a day will help. Run on the spot, do lunges, sit-ups - anything that gets your body moving and your heartrate up. Don't need fancy equipment or a gym membership to exercise. Dance around naked in the house listening to music.
Edit: See you said already that you're exercising. Good. Keep it up.