。
She'll make a kind mother.
Literal
[topic-は] kind mother [into-に] will-become probably.
やさしい ('kind, gentle') is one of the canonical Japanese personality adjectives — broader than English 'kind,' often closer to 'considerate' or 'warm-hearted.' 母親 differs subtly from 母 (haha): 母 is the bare noun for one's own mother used in modest reference, while 母親 ('mother as parent-role') treats motherhood as a category. The conjectural ~だろう makes this a prediction.