。
It's only natural that you think she's coming.
Literal
She [topic-は] come thing [quotative-と] you [subject-が] think [nominalizer-の] [topic-は] reasonable thing is.
もっともなことだ means 'it's reasonable, it's only natural.' ものと functions as a formal nominalizer — 来るものと is 'that she will come.' The sentence validates someone's assumption. 君 (きみ) is a casual second-person pronoun.