Throne of Atlantis is part of the slightly more interesting DC film universe, the DC:AU, which means you can bank on a few elements straight out the gate that score it some automatic points. For one, the animation is going to be top notch, with a consistent look and feel that is extremely clean and modern whilst still feeling familiar to the comic book routes. Next up is the voice acting, which is generally excellent and includes some real star power with the likes of Nathan Fillion as the most perfect casting of Green Lantern ever. Finally, you're at least going to get a competent plot that builds on the films that came before it and slightly leads into future plots, without either feeling the need to ram the movie with easter eggs or to leave a dozen teasers scattered around, neither of which can be said of the live action DC:EU.
On the other hand, it also means having superheroes that are very okay with killing villains, including a Superman that will casually threaten to murder people for mildly inconveniencing him. We're also looking at a Justice League that aren't all that thrilled with working with one another but who, for plot reasons, pretty much already all know who each other are outside of the suits (including having Wonder Woman and Superman dating). Oh, and we're going to get an over-the-top enemy fight sequence on American soil, normally with plenty of dead civilians.
Normally, the balance results in a competent and enjoyable movie, standing heads and shoulders above a lot of the earlier and mid-00s attempts. Unfortunately, Throne of Atlantis doesn't quite strike that balance right. With a reduced running time compared to some of the films that came before it, there isn't enough space to flesh out a whole new civilisation of characters with Atlantis, Manta, the royal family, Merida, and Aquaman himself, plus give the returning cast of heroes much breathing space. The result is a rushed plot that introduces some characters only to kill them off without reason, provides key villain development and exposition that could have been left for a sequel (I'm looking at you, poorly timed confessional of Manta), and an overarching threat that doesn't bear even the slightest ounce of scrutiny.
Here we have the standard Aquaman origin: his half-brother throws a coup and usurps the crown, declaring war on the surface world, resulting in both the revelation that Atlantis is real and the crowning of a hybrid in an attempt to bring the two worlds together. Aquaman and Merida fall in love (in about 24 hours and less than a dozen lines of dialogue), whilst other core characters like Black Manta are introduced and Aquaman meets and ultimately joins the Justice League. But with about an hour of actual narrative time we get a rushed version of events that could have shortcut some areas to give others breathing space. Take the murder of Arthur's mother: why couldn't this have been done at the start of the film, opening on an Atlantis already falling into ruin whilst Manta secretly pulls the strings of an increasingly paranoid tyrant. Merida could have had a more defining role as the person who "rebels" by seeking out an alternative heir. It gives the main characters more emphasis and authority, whilst cutting out a lot of wasted time that has no real pay off anyway.
You also don't need the whole "my father just died" bit for Aquaman. We spend a lot of time on that particular plot point, but it doesn't mean anything ever. Instead, what it continues to reinforce is that Arthur is someone who has only ever known the land, yet at the same time we're meant to believe that he can instantly assimilate into Atlantean culture? It's a stretch and feels like poor writing. As does the invasion plan. For one thing, the army of Atlantis may be technologically superior, but they also apparently only number a hundred or so; a single tactical strike by the airforce would have killed most of that invasion. They can also control and create tsunamis but don't actually use this genuine super weapon at all. We even have Batman stating that they've been beaten and that "people will die", but then they go all Moses on us and part the wave, letting it subside behind them as they... walk across the ocean floor? These denizens of the deep, who prefer to swim, just walked the whole way? And their "tanks" have wheels? Did anyone in the writer's room actually consider any of this?
Speaking of super weapons, Atlantis has a trident that can pretty much kill the entire Justice League. It carves up both Aquaman and Superman like a hot knife through butter, fries Shazam (Shazam!) out of his own powers, kills Cyborg and manages to knock Green Lantern for six. It's also shown to respond to royal mind control, which we know Aquaman can also manipulate, and yet... at no point does he Jedi-mind-grab it out of his brothers grasp? Instead choosing to just dodge it and repeatedly hit his enemy in the head. Also... surprise shark death for Manta! Oh, and even bigger surprise Cthulhu cameo! (That Superman then intentionally kills, regardless of the fact that this is just a creature living in the deep ocean and almost certainly extremely rare). Speaking of which, what on Earth are those fish people meant to be? They thirst for flesh and seem to proliferate, able to walk on land, with sentience enough to understand English yet... have never attacked a surface vessel or coastline? They're practically drowners from the Witcher series but seem content just barely surviving on the scraps of Atlantis. Why?
Plus, one final little irritation is that they clearly skimped a bit on production. There are several purposefully elongated scenes which almost feel like the animation has paused, making certain lines feel like the actors were recording their dialogue months apart but without any editing. That unfortunately means some not-awful jokes are just burned, despite casting some voice actors with incredible comic timing. It definitely feels like, despite all the narrative shortcuts and miniscule run time, they actually had to stretch out the script to fill an hour.
So, basically, this is not a film you need to prioritise. At all.