Joni is a feminine variant of Johnny. Joni Mitchell (born 1943) is the Canadian singer-songwriter widely considered one of the most influential songwriters in the history of popular music.
Subject of David Yaffe's Reckless Daughter (2017).
Joni 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 Joni reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.