From Karen Collins, R.D., MSNBC Health & Fitness:
Those who use frozen diet meals sometimes 
argue that their portion-controlled nature makes them an easy way to get a healthy low-calorie meal. However, they are likely not your best option.
Some such meals have so few calories that they are actually too low for most people, which either encourages unhealthy snacking or, if that does not occur, causes the metabolic rate to slow down, making weight control more difficult.
These low calorie meals are also have too small a portion of vegetables, and a high sodium content.
If you do use low calorie frozen meals, it's recommended that you check the label to choose one low in fat in sodium, add extra vegetables, and don't restrict yourself to them entirely.
