Can psoriasis be fatal?

Almost never. In a very small percent of very severe psoriasis there can be fulminnant cases which require systemic medications. In those cases during flares of erythrodermic psoriasis it is possible to develop systemic infections which in very rare cases be life threatening.
Maybe! Psoriasis is an inflammatory disease. It has now been clearly shown that patients plaque arteries faster, and there is excess cardiovascular morbidity. Psoriasis fatal per se, no-but the immune inflammation causing will causes increase in vascular deaths!