。
She might have been beautiful when she was young.
Literal
She [topic-は] young days beauty was [might-かもしれない].
~かもしれない (might/maybe) expresses possibility — the weakest of the three inference levels: かもしれない < ようだ < に違いない. The speaker considers it possible but is far from certain. This careful hedging is natural when speculating about someone's distant past with limited evidence.