Warner Bros. famously established their DC cinematic universe in reverse, showcasing the superhero origin stories only after their epic team-up.
Wonder Woman proved far superior to Justice League, telling the tale of young Diana, fighting the God of War and walking away victorious. And while Aquaman was also superior to Justice League, the story managed to make the previous film even more ridiculous and nonsensical in comparison.
In Aquaman, Arthur Curry does not fight against Poseidon; he fights his half-brother, and gets pummeled. It’s stated that Arthur isn’t used to fighting underwater, but the implication is that his brother is almost as powerful, or equally powerful, to Aquaman.
Aquaman seems to highlight the fact that Arthur is not unique, in any way other than blood. He’s almost as strong (or weak) as any Atlantean. The very first scene of the film shows Arthur’s mother destroying a room full of super-soldiers, and warrior-queen Mera is just as powerful.
Indeed, it seems as though almost every Atlantean boasts super-strength, making Arthur Curry just another face in the undersea crowd; his only real superpower seems to be to talk to fish. Why was he even chosen to join the Justice League, if there was an entire city of water-breathing superhumans ready to take on Steppenwolf?