Niecy is a 20th-century American invention — derived from the affectionate "niece" and the -cy feminine suffix; popularized as a personal name in Black American communities. Niecy is also an affectionate diminutive of names ending in -niece (Denise, Bernice, Janice).
Niecy 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 Niecy reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.