As Amber Heard told ET Canada, Zack Snyder mentioned that Mera is a warrior queen, which obviously sounds awesome, and getting to have a sword and a crown piqued her interest.
Zack Snyder clearly understood this and attempted to sell Amber Heard on who Mera is as a character.
Amber Heard was not interested in playing the stereotypical damsel in distress that is always waiting to be saved by the burly hero.
We saw a glimpse of Mera the warrior in Justice League, where she did not hesitate to mix it up with Steppenwolf to defend the Mother Box.