These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Cliché Storm: Rebellious human daughter flees overprotective parent, makes deal with evil sea witch to become a mermaid, then that backfires as the bad guy nearly takes over the kingdom. In other words, the plot of the first movie, now flipped!
Fanon Discontinuity: The majority of fans does not recognize it as "canon". In fact, this movie is most often cited to illustrate the "Disney DTV sequels are animated fanfiction" argument.
Idiot Plot: The movie could have ended faster if Ariel had just told Melody why she couldn't go to the sea. While it's pointed out several times in the film, it's after the consequences of it, making at best Better Than a Bare Bulb. Keeping her ignorant of her marine heritage wasn't exactly a smart move either since that ignorance is what sets the second half of the plot in motion.
Moral Event Horizon: Morgana either crossed it when she held a baby Melody over Undertow with the intention of letting him devour her or when she gloatingly left a teenage Melody to drown in an underwater cave, mockingly telling her that "her time as a mermaid has just about expired". In both cases, it shows that she's not above killing children.
The Woobie: Melody in the scenes before, during and after her birthday party.
What an Idiot: Ariel. Even with the reason of being concerned for Melody's safety, you really have to wonder what made Ariel jump to the conclusion that keeping her daughter confined and entirely ignorant of her ancestral roots was the best idea. Ariel of all people should know better, seeing as her own father attempted the same thing with trying to keep her away from land.