It's an idiomatic expression, meaning someone is not naive, innocent, unlearned, ingenious. Usually it is only used together with the verb to become, å bli, but in present tense used as an description of character, å være. (Compare to Hun er ikke dum = She is not stupid).
Hun ble ikke født i går = Hun er ikke født i går. Whether to choose ble or er is much about dialect.
It is said in Italy too! Fancy two extremes of Europe using the same idiomatic expression
In the Netherlands as well, in a slightly different form: she isn't from yesterday