In this movie, Nobita and Doraemon accidentally strand a mermaid princess in their yard. I distinctly remember this being the first Doraemon movie that I actively thought was bad, and sadly, my opinion has not changed much after a rewatch. I wish it weren't so, because the undersea setting and mermaid concept could have been a lot of fun.
To briefly discuss a few of my main issues with this movie:
-The mermaid princess Sophia is amazed by cars and doesn't realize they're dangerous, yet we later see that she comes from a society that has roads and vehicles.
-Dorami suddenly shows up to deliver some exposition about mermaids, apparently at Doraemon's request, even though mermaids have barely been mentioned by that point.
-When the villains are about to invade, the mermaid queen dumps the responsibility of leading the war onto Sophia, and this is portrayed as a good thing for some reason.
-There's an abrupt scene cut into the middle of the final battle. (I seriously wondered whether I'd been watching a faulty copy of this movie when I first saw it.)
-After everyone spends much of the story trying to figure out how the obtain a legendary mermaid sword, the end result is that Sophia can just summon it without even intending to, because she happens to be the chosen one, rendering most of that subplot pointless. (Also, why couldn't the protagonists use Translation Konjac to decipher the ancient texts about the sword again? They've done similar things in previous movies.)
-Despite that, Sophia ultimately doesn't do much to help neutralize the villains. The sword she summons immediately gets taken by the villain leader, making him more powerful, and it isn't even directly used to defeat him later. The other main thing Sophia does during the final battle is attempt to fight the villain leader herself, and... she gets disarmed in one hit. Yet her actions in battle are enough for her to be crowned queen in the aftermath. Huh?
-The porcupinefish-boy Haribo (who is somehow the leader of the merperson army) is a leading contender for the most annoying character in a Doraemon movie.
-As if all of that weren't enough, this movie includes one of my "favorite" recurring Doraemon tropes: Shizuka getting captured and needing to be rescued. As far as I could tell, this doesn't even serve any narrative purpose here other than to add some tension before the final battle begins. The rest of the story probably would have played out essentially the same way without that subplot.
This next observation is not necessarily a flaw, but Nobita's Great Battle of the Mermaid King is another movie in which the main characters seem rather unfriendly towards each other, more typical of the main Doraemon series than of the franchise's cinematic lineup. Gian and Suneo shamelessly bully Nobita in the beginning of the story, and even Shizuka doesn't show any sympathy or concern for him upon witnessing this.
There is also a strange scene in which everyone else chews out Suneo for being unable to point out a specific constellation (which they suspect to be the planetary system where Sophia's people came from) out of hundreds if not thousands of stars in the night sky. They apparently take this as an offense to Sophia's feelings, though Sophia shows no signs of being upset by it.
To be absolutely fair, I don't think this movie is entirely devoid of merit: the underwater scenes are often quite pretty, and it's nice to see Dorami get screentime that amounts to more than a voiced cameo. If I had to pick a least favorite Doraemon movie though, this would almost certainly be it.
