In First Class, assuming that older Mystique isn't a real person in that universe, Jennifer Lawrence's Mystique is able to transform into Rebecca Romijn, her "future self". This implies that Mystique has full control over her looks. She is not limited to copying someone, but can create new appearances by imagination or observation alone.
First Class and Days of Future Past are in continuity with the original X-Men trilogy as far as cast/basic looks, as noted by the end of DOFP. In X3, Mystique is depowered by the Cure, and reverts into her normal human unaltered form (Romijn), not just what she thinks her original human form looks like.
In universe, this shows that her younger self could transform into a person that does not yet exist.