Mariella is the Italian diminutive of Maria — from the Hebrew Miryam (beloved, bitter, sea). Mariella Ahrens (born 1969) — German actress. Mariella Aldobrandini — Italian historical noble. Princess Mariella di Pierfelice — Italian aristocratic naming.
Mariella 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 Mariella reduce to 8, The Visionary. This is a traditional interpretive system, not a factual claim about the name.