Yes, Ariel was a redhead with blue eyes and pale skin in the original. No, that doesn't mean all mermaids have to be redheads with blue eyes and pale skin. There isn't anything about Ariel's character that requires her to be white--"The Little Mermaid" isn't about whiteness, it's about a bratty teenager who exchanges her voice, family, home, and fins for a boy she barely knows.
While Twitter has been buzzing about Disney's casting choice for awhile now, and most of the response is positive, some people simply cannot imagine a universe in which a mythical creature could be anything but white. Race doesn't exist under the sea, people! There aren't any humans under the sea!
Many people noted how important this moment would be for girls of color: