In general, I think all the first movies were a ton better than any of its sequels. The exceptions would be Toy Story 2, Toy Story 3, and Monster's university. I thought the Lion King sequels were clever, but the original was still tons better! Usually, they're just not as funny or original!
A sequel is fine if they have great ideas for its plot. In the case of The Little Mermaid 2: Return to the Sea, I felt like it was really unnecessary. It was so flat and less magical than the first. It felt like it was a carbon copy of the original and didn't have something new to offer to the viewers. Also, they should have had Melody's eyebrows trimmed, even a bit (haha), she's supposed to be a pretty girl because her parents are both good-looking!