Alas, based on her recent comments about abortion, I would have to say that no, she's no longer Catholic. She upholds and promotes not only the liceity of abortion, but even declares it to be some kind of fundamental right of a woman.
So, one can be a heretic on any matter of faith and morals, and the grave immorality of abortion is dogmatic in the realm of moral theology via the Ordinary Universal Magisterium. Sometimes it's unclear whether the OUM has defined something, but abortion has always, everywhere, and by all Catholics been considered a grave sin.
Conqeuently, Melania Trump is a manifest heretic who's no longer a member of the Catholic Church.