What it means
のももっともだ expresses that a situation or reaction is only natural, reasonable, or understandable in light of the circumstances. It presents the speaker's judgment that there is a good reason for something to be the case. The form usually follows a plain predicate, and it is somewhat formal and more common in explanatory or written language than in casual conversation. The polite counterpart is のももっともです.
Usage note
Do not confuse もっともだ here with the superlative meaning of もっともだ as “the most.” In this construction, it means “reasonable” or “understandable.” With nouns and na-adjectives, な is required before のももっともだ.
How to use it
その学生が初心者なのももっともです。
It is only natural that the student is a beginner.
彼が不安なのももっともです。
It is only natural that he feels anxious.
彼女が寂しいのももっともです。
It is only natural that she feels lonely.
彼が驚くのももっともです。
It is only natural that he is surprised.
その学生が初心者なのももっともだ。
It is only natural that the student is a beginner.
彼が不安なのももっともだ。
It is only natural that he feels anxious.
彼女が寂しいのももっともだ。
It is only natural that she feels lonely.
彼が驚くのももっともだ。
It is only natural that he is surprised.