Using Jessica Alba is like the worst example, the worst case scenario.
Its like pointing at Kate Maras Sue Storm(Weird both examples are about sue storm)when being worried about wigs.
They two simply had unfortunate bad luck in the hair department, generally its fine and i think it would work giving her blonde hair.
I dont know, i question how those who complain about her not having blonde hair really care for the character when the hair color is where they draw the line.
Seems more like people mostly only care for superficiality if the be all end all is Supergirls Hair color.
And this is even a special occasion because it is a movie about an alternative Universe, so we cant even be sure if that will be her look after Flashpoint.