The short answer is yes, this is because any kind of exercise will help you burn fat and therefore, lose weight, you only have to make sure to eat right and don't let the Karate do all the work for you.
Any exercise will help you lose weight. Cardiovascular exercise that focuses on the thigh muscles (because they are the largest muscles and burn the most calories) are the most efficient. If you are doing active karate moves, it should be very helpful for weightloss.
Karate training can be a very strenuous exercise. All strenuous exercise can help you lose weight. Karate alone, without a diet plan, may not do much, but it is just as good as most other exercises.
