I might be wrong about the term, but I'm talking about the thing that goes like this: "When I was a child, my planet Krypton was dying. I was sent to Earth to protect my cousin". Towards the end, she says "...to protect my city from aliens and anyone that means to cause a harm". Obviously, she is an alien herself and again obviously, there are nice aliens and there are not-so-nice aliens. Why would she say that and generalize all aliens despite she is one?
The actual quote is
But in secret, I work with my adoptive sister for the DEO to protect my city from alien life and anyone else that means to cause it harm. I am Supergirl.
So she doesn't generalise ALL aliens she's referring to aliens that mean to cause harm.
The emphasis is on mean to cause harm ...not on who/what is causing the harm.
Arguably, this could come down to punctuation...add a couple of commas and the emphasis becomes clearer.
But in secret, I work with my adoptive sister for the DEO to protect my city from alien life, and anyone else, that means to cause it harm. I am Supergirl.
Additionally, it's not certain that her city (D.E.O. notwithstanding) has the capacity to deal with aliens at all.
Even well-meaning aliens can cause difficulties and she is, perhaps, better equipped to deal with them that a normal human.