the attitude from what is it, 90 seconds of footage
I don't know what version of supergirl this one is based off, but as I said, supergirl isn't just a gender swapped superman despite what the original film did with the character way back, she's a character that was a generally "normal" girl who presumably liked to have fun with friends, then her world literally fell apart around her and she appears to have fallen off the tracks and into the drink, a bit more depth to it than "i'm a prettier superman, with boobs".
She's not the "all American hero", she wasn't raised with those powers and taught to be the American ideal, she is/has been in mourning and coping badly with it in a way that is quite human.