Like Niobe, all tears:
Niobe is a character in Greek mythology, who boasted of her fourteen children to the goddess Leto, who only had Apollo and Artemis. Because the Greek gods/goddesses despised hubris, Leto sent her children to kill all of Niobe’s. Niobe climbed to Mt. Sisyphus, and was turned to stone, from which tears poured unceasingly (there’s actually a rock outcrop with the appearance of a woman’s face, see the picture below, associated with Niobe since antiquity).
Hamlet is comparing his mother’s grief to that of Niobe. This is meant to be sardonic — even if his mother cried an absurd amount like Niobe, not a month later she was already married to her late-husband’s brother.
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