Not entirely sure why it is even relevant who Zeus's first mortal consort was, but apparently it is since several authors make a point of mentioning it:
Diodoros of Sicily, Library of History 4.14.4: „The first mortal woman, for instance, with whom Zeus lay was Niobê, the daughter of Phoroneus, …”
Dionysios of Halicarnassus, Roman Antiquities 1.177.3: „Pelasgus was the son of Zeus, it is said, and of Niobê the daughter of Phoroneus, who, as the legend goes, was the first mortal woman Zeus had knowledge of.”
Pseudo-Apollodoros, Library 2.1.1: „Argos was the son of Niobe and Zeus (she was the first mortal woman with whom he had intercourse). According to Acousilaos, Pelasgos was also their son and the inhabitants of the Peloponnesos were called Pelasgians after him.”
Pseudo-Hyginus, Fabula 145: „From Phoroneus and Cinna were born Apis and Niobe. The latter was the first mortal Jupiter slept with. She gave birth to Argus, who gave his name to the city of Argos.”
