Judy is the English diminutive of Judith — from the Hebrew Yehudit ("woman of Judea, praised"). Judy Blume (born 1938) — American author whose young-adult novels Are You There God? Judy Chicago (born 1939) — American feminist artist; her installation The Dinner Party (1979) is among the most visited works of feminist art.
Judy does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Judy reduce to 6, The Nurturer. This is a traditional interpretive system, not a factual claim about the name.