As he told Business Insider, Aquaman is the character that gets made fun of; he’s the one who talks to fish, wears an ugly suit and was the subject of a storyline on HBO’s Entourage.
So in theory there were no expectations and nowhere to go but up.
You can see the logic in James Wan’s thinking here.
Aquaman is far from the most popular DC hero and fans don’t have the kind of built up expectations for him that they do for the characters that have had multiple cinematic outings and enjoy greater popularity.