Although a Christian, she displays the qualities necessary to be a Bodhisattva. Karuna is not exclusive to Buddhism and she 'represents love without distinction'.
The Bodhisattva ideal cannot be limited to Mahayana Buddhists as that would contradict the belief that the Bodhisattva's love is both universal and impartial.
It is not impossible that Mother Theresa was a Bodhisattva who manifested in. Christian form to propagate the ethic of compassion.
Principles of the religion are more important than the religion itself.