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.
Pondering the meaning of this expression in English, there is some reason for the Norwegian use of the present tense. "Yesterday" has a double function, to express prior time, but also (via the negative) a quite recent prior time in order to convey an actual birth sufficiently long ago to allow time to develop the understanding that experience provides. In other words, the Norwegian sense of not being born recently merges with not being born at the present moment. If we think of the function of "yesterday" in the English expression, it is a simple step to think of approaching the present moment and use of the present tense.