Are lung blood clots fatal?

Not always: Pulmonary emboli (blood clots that travel to the lungs) vary greatly in size from patient to patient; some are trivial, others are rapidly fatal, and there are all sorts of outcomes in between. They are usually a serious problem and need prompt and effective treatment, and diagnosis as to where they come from, so that can also be treated. Anticoagulants ("blood thinners") are usually prescribed.