Mariel is a modern English elaboration of Mary — from the Hebrew Miryam ("beloved, bitter, star of the sea") — possibly influenced by the Spanish Mariel (a coastal town in Cuba). Mariel of Brittany — Arthurian legendary princess. Mariel Margaret Hamm is Mia Hamm (born Mariel Margaret Hamm) — covered separately as Mia.
Mariel 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 Mariel reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.