This is funny because my friends and I used to call Chaotic Evil "Stupid Evil" when we played D&D

It's a matter of opinion.
Chaotic Evil gives you a fair amount of freedom, it's in the interpretation.
Niamh doesn't kill everyone she sees, but she will randomly decide that she does not like someone or a group of someones, and then all hell breaks loose.
Fundamentally it means you can't trust her one little bit because she will hack you to death as soon as look at you, but she might not for an equally random reason. In every case what she will do is try and get whatever she can for herself, if she thinks that you have something more to give she will either not kill you because she thinks she can get more, or she will kill you so she can take it. Conversely if you have nothing she might just kill you anyway because you may be a threat depending on context or because you are no longer of value to her.
She is not remotely interested in whether you are of value to anyone else, she is only interested in what she herself can get. Such consequences as there may be - bounties, assassins, inability to speak to people - are without meaning or significance to her; simply - the consequences of her actions are inconsequential.
About the only species that do not become targets are bunnies and other elves, unless of course they get in her way or attack her first - (neither of which happens with rabbits).
To put it another way, if you met her in a tavern and ended up sharing a bed, you would have to make damn sure that you were good for it, because your life would absolutely depend upon your performance!
Or not.