There certainly are foods that you can consume in larger quantities due to them being ‘diet’ however, always pay close attention to the actual nutrition facts.
The terms ‘diet, low-fat, low-calorie, zero calorie, etc’ when reading food labels, are dependant on what that food is compared to. Something that may be low in calories, might be packed with fats. Something low-fat might be packed with sodium. Something zero-calorie might be packed with preservatives.
And nothing is ever good without moderation because anything can be unhealthy in large quantities.
So, always remember to read the information on the package and moderate.