Oh, look, it's the Star Wars fandom circa 2016. People with no idea what Mary Sue means bandying the term at any female character who does anything noteworthy.
There was no Nelf army at Teldrassil -- only the city guard. They say she didn't want to burn it at first -- only to capture it so that Alliance won't have easy access to Azerite. But on the other hand, she enhanced the trebuchet with the help of shamans beforehand, which means she planned it all along.